At the start of every month, millions of hopeful savers up and down the country check in on their Premium Bonds to see if they have come up trumps in the monthly draw. Have they won £25, £100 or even £1,000?

Winners find out about their prizes via email or on the National Savings & Investments smartphone app – except, that is, for the two lucky people who win the £1million jackpot in each draw.

The lives of two Bond holders are transformed on the day before the first working day of every month, when NS&I personally delivers the news 48 hours before the other prizes are dished out. From joy and relief to disbelief and denial, winners react in all manner of ways when they learn of their newfound wealth.

How do I know? Because I'm one of the five NS&I employees – known as Agent Million – who break the news to lucky winners.

If you get a knock from me on your door, it means your life is about to change. I've been visiting top prize winners in my coveted role as Agent Million for almost 11 years.

Each month, one agent travels the country armed with a few details about the winners, ready to break the wonderful news.

Premium Bonds are savings products that offer the opportunity to win monthly prizes instead of a guaranteed interest rate. Savers can funnel between £25 and £50,000 into Bonds. Each bond, worth £1, enters the prize draw each month.

Ernie – the Electronic Random Number Indicator Equipment, which randomly generates the numbers of winning Bonds – is very popular. He even gets Christmas cards!

And as we know a little bit about the winners from the details they filled in on their accounts, such as their date of birth, we can put them at ease when we break the news and prove we are who we say we are. But delivering the news can be tricky, as we need to be completely anonymous – to their friends and family, as well as curtain twitchers.

If our faces are known to the public and we are spotted knocking on a door, winners might not be able to keep their windfall a secret.

The main reason we keep our identity under wraps is for our safety and that of the winners. It's important we're not spotted going to their homes, making it obvious someone is newly wealthy.

Being completely anonymous also means winners can decide who they share their news with. One lady I visited didn't even want to tell her husband about her £1million win. She was alone at home when we visited and didn't break the news to him for ten days. She said: 'I just wanted to enjoy the moment.'

Husbands and wives can be quite competitive with each other when it comes to their Bonds – and hitting the jackpot is the ultimate triumph. One winner's husband checked their bonds each month and would tease her if his savings yielded more money.

After I had broken the news to her, I suggested she opened the app that month instead of her husband, and showed him the fireworks which appear on the screen when you win the jackpot.

When a winner answers the door, they might assume we are selling something. I reveal that I am a customer service manager at NS&I but that's all I can say. If someone other than the winner answers the door, we can't tell them why we're there.

In one memorable case from my early days, I visited a winner's home and his wife answered the door. I was told he was out and would be back at 6pm. I said: 'I'm sorry but I can't say anything, I'll call back when your husband is in.'

I visited at 6pm but he still wasn't back. The wife, who was in her garden watering the plants, started to question me, asking: 'Why do you want to see my husband?' She threatened me with the hose pipe and I narrowly avoided a soaking!

I came back half an hour later and he was home. As soon as she knew who I was, she was mortified and said she'd jumped to the wrong conclusion.

She referred to me after that as her guardian angel.

Our discretion continues even as we're leaving a winner's home to hide the news from neighbours. We might say something like: 'See you soon. I won't leave it so long next time!'

In one case, a lady who lived next door to our winner was in her garden reading so I couldn't say my usual line about being a customer service manager at NS&I. I simply asked: 'Can I talk to you?' Unsurprisingly, she was reluctant to let a stranger into her home. I then said, very quietly: 'There's a lady in the garden next door but I'm here to talk to you about your Premium Bonds.'

One of the great joys of the job is that we see winners from all ages and walks of life. I've visited three-year-olds where I've had to walk over Lego pieces while trying to talk to their mother, but I've also met very senior people.

I've even seen teenagers who have won the top prize. One boy, whom I will call Shawn, had been on a night out with his friends the day before I visited. I knocked on the door of his family home at about four in the afternoon to deliver the good news – but his father told me he was still in bed.

Shawn eventually got up and I reassured the family there was nothing to worry about. I told him he'd won the £1million jackpot and he looked at me in total disbelief.

He didn't have his mum and dad in the room with us so I asked if he wanted to get his mother. He immediately said: 'Yes please!'

Shawn had bought the Bonds with money his late grandmother had left him. It was her heaven-sent legacy that turned him into a millionaire. He wasn't flashy, either. He had just learned to drive so he very sensibly used his winnings to buy an old car.

Surprisingly, we don't often hear about winners splashing out immediately. The day they win, they may say they'll buy a bigger house or a flashy car. But when Sarah – the manager in charge of all Agent Millions – speaks to them the next morning, their exciting Plan A has become Plan D.

Not all winners are sensible, however. I recall one who wanted to drive past the Aston Martin garage to pick out a car.

Above all, most winners want to help their family. Perhaps they want to pay for their children or grandchildren to get on the housing ladder or pay for school and university fees. Others relocate to be closer to their family.

I've seen a terminally ill winner who used the money to make sure their family was set up for life.

The £1million puts people in a position to be comfortable – it is just the right amount.

There have been a few occasions where a winner's reaction has stumped me. When I told one lady she'd won, she said: 'I don't want it.' I was gobsmacked – it's the most interesting reaction I've had from a winner.

I said the money was hers but she still replied: 'I don't want it.' She eventually sent it to charity.

She wasn't the only winner not to want the £1million. One person said their life was set up perfectly fine, their finances were ticking along, and they didn't have to think too hard about anything.

Suddenly, they had this £1million pot to consider. Sarah said once the winner had slept on it, they decided they did want the money after all.

I stress that winners shouldn't spend the money until they have spoken to a financial adviser.

When I'm not delivering the good news, I work in the call centre and in the prize department. Telling someone they are now a millionaire is a privilege – I would love to do it every day, if I could.

• Names have been changed to protect identities.

                                                                                                                         As told to Lucy Evans