At long last,I can now say my lap trigger works perfectly!
The problem is that,realistically,the only place to mount it is on the top yoke(for a clear signal and in order to press the button once for startline location).Unfortunately,my camera is also on the top yoke which,for some strange reason was upsetting the gps trigger.
Obviously,this is not a problem if the camera is somewhere else.
I have since lengthened the wire and mounted the trigger on the back of the seat,with excellent results,although this is slightly inconvenient when needing to set the startline at a different circuit(pressing in the pitlane when first going out appears to be good enough).
It is certainly a useful addition for recording the info on the dash and beats flicking the headlamp flasher!
Obviously,being only a trigger,it has no memory other than the co-ordinate set by pressing the button once on the startline.Future laps will then clock up each time that point is passed.
It will remember that co-ordinate if your next visit is the same track.For a different track,just press the button at the startline.
It takes a few minutes to locate the satellites first thing(not usually a problem whilst warming up or noise testing-or just leave the ignition on),it is then usually ready to go for each session.
Of course,it could be better,a longer lead and a remote button would be an improvement but,for the price,it does the job.(An even better improvement would be for bmw to add a gps trigger in the first place!).