UK Plummers, how much would it cost to replace an indirect water cylinder?

The only way is to get at least three separate estimates. I realise that cost may be an issue, but if you can possibly stretch to it buy a quick recovery cylinder. They are smaller than standard, but reheat incredibly fast so saving energy as the boiler runs much less. I fitted one recently and from stone cold it gets piping hot in 15 minutes. Because they work so well you can save space, as they are about half the size of your original cylinder.