Thinkin about it, I think that maybe the police officer says 'he's spotted me' on puropse for this very reason. As up until that point it can be argued that the rider is riding as he normally does having shown no overt signs of clocking him... and then upon spotting the officer is riding for 'the game' of it, so the evidence after this point would have to be stayed...
I can't say I'm a legal beagle though was just thinking...