In reply to Bjartur i Sumarhus:
If you're doing it yourself, you have two options. Either get a registered HETAS fitter to check that you've put the liner in correctly, connect it up, register plate etc. and give you a filled in certificate. He will also fill in the safety plate details which has to fitted either near the fire or next to the consumer unit. This is a legal requirement under building regs, as is fitting a CO monitor in the room. Your other option is to register the proposed work with the local council and have a building inspector pass the work. He will want to see it before the liner goes in, and will then tell you what he wants to see next visit. Usually to make sure the lintels are ok, and liner is the correct way up. Final visit will confirm correct connection, air gaps from walls and combustibles and that all building regs are complied with. Then you get your signed stuff.
Without either of these, your house insurance is invalid and when you sell, without a certificate it's likely you'll need to disconnect it and declare it decommissioned.
I have a wood burner arriving tomorrow which cost a grand. The fitting kit and liner cost 350 less a 150 discount. Worth paying for the best quality liner. I opened the builders opening myself, put in a new hearth, and lined it with cement board. You can't use plasterboard. A HETAS registered builder is fitting the liner, fire, commissioning it and supplying certificate for 300.
Grand total is 500 plus fire, although it would have cost an extra grand to have the fireplace opened up and made good for the builder to do it.
FYI got a bespoke fire surround in green oak made to measure for 400 so just short of 2 grand for the lot