Cheers, I hadn't looked on Toolstation. Although I really need a definitive answer from building control on how many smoke alarms I need, 8 seems excessive for a 3 bed flat but details on what you need if you don't have fire doors is scarce.
Have a look at Aico/Ei gear - several different ranges, and radio link bases so you can make the alarms talk to each other where running cables isn't going to happen. Ours are powered from the local lighting circuit, with long-life battery backup, so all I had to do was run a couple of feet of cable above the ceiling from each light fitting.
The installation guide on their site is pretty good in terms of suggesting where you should site them. (But yes, eight sounds about right if you're putting one in every circulating and living space - 3x beds, 2 or 3 in hall/landing/stair, 2 or 3 in kitchen/dining/living.)
As for suppliers, the SafeLincs site is reasonable - rarely the cheapest, but fairly competitive, and lets you see a lot of ranges in one place. (I think it started as a trading arm of a safety partnership including the local fire service, so again, they're pretty good on advice.)
Mind you, I think we got our alarms in dribs and drabs from a bunch of ebay sellers, and saved a fortune.