I've got a water meter - it really is the cheapest option, swapping from an unmetered supply has saved us hundreds of pounds a year.
Yes you will pay the £75, but if you don't use any water, that is all you will pay. Doesn't paying £75 seem more attractive than paying £2K for water on an unmetered arrangement? To me it's a no-brainer, and it's only putting off the day when getting a water meter will be mandatory.
You can't not pay now for charges you have incurred in the past two years, you owe it you pay it, you don't want a CCJ on your record, it'll give you all sorts of problems in future.
My recommendations therefore are 1) pay for what you've been charged to date, and 2) get a meter a.s.a.p. That'll prove how much you use (or don't use) in a year, and you only pay for that.