flush mount alarm in celing pir

screwfix

 

Most of the flush units around are duel tec's primarily for commercial and would be a bit to big for a domestic.
Try this
http://www.gesecurity.com/portal/GESDownload?ID=118&DID=1573&documenttype=DataSheet

Should sort you out

 

FLUSH MOUNT i domt want it to look like ther is a
ashtray stuck on my celing

I know what your saying "flat with the ceiling" I got one from CEF a few years ago, but my latest ones have come from a wholesaler called Furnells, give them a ring 0161 973 6060, they're in Manchester (dont know where you are) but are quite helpful and will probably tell you the make of them, they pick up excellent as well, ask for Darren.