Stephen Hester, chief executive of RBS (which is 83 per cent owned by the taxpayer), lives in Holland Park, where one-bedroom flats get snapped up for about £1m. Last week I paid him a visit to express my disapproval of the financial assistance his bank provides to companies that make cluster bombs. I dressed up as part of an explosives-disposal team and wrapped his house in danger tape.

I was asked by Amnesty International to make a short film about how several of our high-street banks are profiting from cluster munitions. The chances are that you have an account with a bank that has been secretly making money out of several US companies named and shamed by Amnesty for continuing to make the ghastly devices. Barclays, Lloyds TSB, HSBC and RBS will continue making money out of the companies unless we stop them.

The really awful thing about cluster bombs is their longevity. They kill and maim decades after they were dropped, which is presumably why Israel deposited millions of them in the last three days of its Lebanon invasion in 2006.

I wanted to know what effect they would have, so I went to Laos, which has the unhealthy honour of being the most-bombed country in the world. For 10 years during the Vietnam war, the US Air Force repeatedly stamped on this unfortunate population, depositing several hundred million cluster bomblets on to it.

It is estimated that 80 million of them still remain, of which one-third are still dangerous. I won't deny that I kept that information to myself when booking travel insurance.

I was introduced to "Buffalo" Bob, the operations director of the local Mines Advisory Group team – an NGO that runs cluster-demining programmes all over the world. Bob was a swarthy and immediately likable bloke, and showed me the scars he'd received from "playing" with a cluster bomb when he was a teenager. The experience nearly killed him, but also set him on the path to dedicate his life to ridding his country of them.

We went out to observe a search team in action, which spends its days in the blistering heat combing paddy fields with metal detectors. Even today, 300 people a year are killed in Laos from cluster bombs, even though the fighting stopped nearly 40 years ago.

I spoke to one member of the search team who had lost a leg several years ago when he had been farming in his garden and set a cluster bomb off. Given the standard of healthcare in the country he was extremely fortunate to survive, and the first thought that went through his mind was "How will I be able to look after my family?" This answered a question that had been on my mind since I heard about the problem – why don't people just stay away from the land with bomblets on it? The answer is that they'll starve if they don't. Having witnessed the appalling effects of cluster bombs first hand, I was furious as to why our banks, especially the ones we own, thought they could turn a coin from making new ones.

So I called all the banks to see if they would care to discuss their ethical values on camera. Unsurprisingly they all told me to piss off.

Undeterred, I marched off to RBS headquarters on Bishopsgate. Most of the bankers were on a direct stagger from their publicly owned desk to the pub. For the few who did bother to speak to me it was to tell me what I could do with my clipboard. Finally one of their merchant bankers had a chat:

Me: "Did you know that RBS are investing in the companies that make cluster bombs?"

Banker: "Excellent – how much money is it making us?"

"About £20m," I guessed.

(Groans with pleasure) "Print that."

"Is that good for you?

"Yep. How do you think you get paid your benefits?" (I was wearing a pretty grubby T shirt.)

"Er, I'm not on benefits. I'm a journalist."

"How do you think the banks make money? By investing in things."

"But cluster bombs mainly kill civilians."

"They're 'things'."

The effects of cluster munitions are horrific and most of our high-street banks are assisting their production, in direct contravention of an international treaty that Britain has signed.

If we the public tell the banks that they have to stop or they will go the way of the News of the World, they will stop stonewalling and end these murky dealings on the spot. Contact your bank to tell them your views on this and they will have to act.

Chris Atkins is a London-based film-maker. To watch his report on cluster bombs go to www.amnesty.org.uk/rbs