Installing double glazed windows in a flat

[LOVELYDOVEY05]

Does this have to be done by a Fensa registered person/company?

I don't see why it would be different to a house, so yes, fensa or certass certified person, otherwise I think you'd need building regs sign off

Most windows companies are FENSA registered anyway?

Depending on the type of flat and the management and freeholder set ups, it may not be as simple as just getting them replaced yourself.

The guy recommended to me is Trading Standards approved by the County Council so is that ok?
Yes Wowfudge good point. The flat is freehold so no problem but I know in some leaseholds you have to ask permission of the Freeholder. Otherwise a sale could be held up

I'd double check on the fensa and certass websites that they are on there to be sure, but I'm sure they will be if they're trading standard approved