I had an electric meter that used cards, £20 lasted a month, the landlord came the other day to replace the electric box with one you can top up online, £12 has lasted a week, he said it would be the same costs, what has happened?

Yeah, my background is in energy supply and I am already fairly weary about sub-metering personally. Energy supply is already confusing for a lot of people without adding sub-metering into the mix. I get the idea of sub-metering when it comes to say HMO or short term lets for example. Landlord saves money by only having one supply rather than paying thousands for additional supplies, and tenants can pay for their share of the usage for the whole property. However, this system is open to abuse as you pointed out - especially with people who aren’t savvy to how it all works. They take the landlords word that bills are included and give little thought to getting proof that their costs are what they are being told. There’s also the fact that private metering is a minefield. There’s no guarantee the sub-meters are good quality and there’s no real regulation/protections around them that you’d get with supplier meters. If a supplier meter is faulty, they have to bill you fairly by law (and can only back bill a slow meter up to 1 year). With a private meter it’s up to you to dispute with the landlord. I own, but I have friends and family who have been in this situation and I always tell them to get unit prices and copies of the monthly/quarterly usage off the landlord. Landlord increasing the price per unit on a sub? Ask him for proof that the supplier has increased the unit price on the master meter first. Like you said, the opportunity to scam people is there and it’d be easy to do so considering a lot of people just take their word for it. Just because it’s illegal, doesn’t mean landlords wont try it anyway.