don't buy them from screwfix, they are expensive!!
also under the Regulatory Reform (fire) Orders 2006 these have to be supplied and fitted by a qualified person preferably under FIA or BAFE approved.
especially if your property is required to have them!!
look around and chat to a few people that you work around/with and get a few quotes, do not use Chubb or the large multinationals they will charge you a fortune!