I fitted a Cyclone v2 which cost me a whopping £30 or thereabouts. Piece o piss to fit. Bloody loud and adjustable sensitivity with a tidy wee keyfob. It's NEVER gone off on it's own by mistake, never drained the battery and has deterred thieves with a 2 stage warning. Very bright blue led to warn folks off. It's not thatcham approved but it's miles better than both spyball and crappy datatool alarms I had which went off if there was a programme on telly about thievery or if you dared to leave it alone in a garage (maybe it was scared?). It's not as tidy a package but it fits up under the tank quite nicely.
http://www.motorbikealarm.co.uk/The aftersales service is excellent as well.
