I've just bought a Yale wireless alarm system for my house and have been looking for a few add ons. Here's what I found on Tescos website (no idea about instore).
Keypad £14.68 was £29.36
Remote Controller £14.68 was £29.36
Smoke Detector £14.68 was £29.36
Door/Window Contact £9.78 was £19.57
Don't forget p&p, but quidco and vouchers you could get more money off.
(The £7 off £50 worked fine for me)
Hope this helps someone :)
Hope it helps