To quote from Thames Water on their Assessed Household Charge

This charging system was agreed with our regulator Ofwat in 2007, came in as of 1 April 2008, and the Single Occupier tariff later came in as of 1 April 2009. Properties are placed in a band, based on the number of bedrooms at your home, or a single occupier tariff if you live on your own.

Now I was assessed in late 2008 and placed in band 1 and have been billed band 1 ever since.

Only recently did I discover that the single rate also exists so have applied for it and been accepted.

Now luckily I have a copy of my original meter assessment form (completed and signed on the day of my assessment) which states on it 1 bed 1 person..........

I can't remember a bill or leaflet that advertised that they had brought in a lower rate I was eligible for or I would have applied for it.

So the question is since they had the information that I was eligible and failed to adequately advertise the change to me........

Would this be worth pursuing?

I know with benefits etc that them having the information but not applying that to future changes is an easy win at appeal but what about this scenario with Thames Water?

It could mean a refund of around £700

note the word OR. I suspect that there will be no change as my presumption is that a larger house with a single occupant would benefit, not a single bedroom property as this is assessed at the lowest rate anyway

Nope the single occupancy is lower than the 1 bed rate. About £80 lower this year. iirc un assessed and un metered are all placed on the 3 bed rate by default.

I've already applied and been accepted on the single person rate so this is about getting it backdated as far as possible.

I'll write a speculative letter/complaint and see what they say. I doubt they'll agree but at least once I have their reasoning and a deadlock letter I can bump it to OfWAT or whatever for an opinion.