Unvented cylinder needs 22mm - water meter issue?

[CBF]

you also need to make sure the plumbers hold the G3 cert for installing un-vented cylinders not just any plumber can install these you need to be qualified.

Will do, I didn't realise they needed a special certificate! They were recommended by the builder who's doing the main renovation works and we've seen a similar system that they installed in another house. They're from a reasonable sized plumbing firm (i.e. not just a one man outfit) and they also sent a different guy who was LPG certified since our boiler is not on mains gas. Presumably they'll know about these "G3" regs so I would hope they are using the right chap. I'll check anyway.

if it's the gas safe guy installing the cylinder it will be on the back of his gas safe card, if not they will have a card from which ever cert board they did the course with, plus it must be re-newed every 5 yrs (it used to be once done it lasted for ever) so if they did the course/exam before oct 2007 then their cert has run out so you would either need someone else or BC to sign it off.