I have a Micromark Wireless system which works very well indeed. It has all the features of a wired system and is very easy to maintain - changing batteries in the PIR and door/window sensors about every 9 months. The siren situated outside on the wall runs off batteries but these are topped up by solar power, even if the sun is not shining, and need changing about every 15 months. I fitted it all up myself and I'm no electrical genius.
The basic kit came with 3 PIRs, 2 door/window sensors, a fixed keypad and a remote keyfob plus the main siren and was £99. You can easily add more sensors if you want.