<<The ATC radar sweeps round about every 2 secs. That could cause your detector to go off every hundred yards?>>
Close, but not quite - it would take off! The big one by the tunnel sweeps about every 5 seconds if memory serves me.. i.e. 12 times per minute. The surface movement radar is much, much faster but does not have the same transmission power as the big boy... (Yet again I look to current colleagues to put me right!)
Irrespective of what detector it it, it's probably of very poor quality and wide open to anything so radar could be the cause. I'm a radio amateur and could tell you tales all night of equipment which receives radio signals but isn;t intended to - like electric kettles, hi-fi, deaf aids, church bells, etc!