Mine's mounted on the swingarm / chainguard (one of those screw together with Allen screw types and never had water in yet.
Bludclot: " i bet there's only a tiny number that are actually stolen from motorcycles and should it happen and the bike owner takes the proper steps of reporting and re-applying then there's no monetry cost involved." - I had a tax disc stolen some years ago from a cheap runaround 'ped during my L-plate days (annual tax £15), replacement disc £7 (about 6 months worth of tax, my tax at that point had between 2 and 3 months left on it).
Tax disc thieves aren't your "typical" thieves looking to cash in (hence why they don't nick tasty Yoshi cans), they'll stick it on an untaxed vehicle (so that from a distance, the untaxed vehicle "looks" like it's taxed because it has a disc (and such vehicles are likely uninsured or without MoT).
I just paid my tax for another 12 months, it took DVLA about 5 seconds to verify the bike's insurance / MoT details (those who don't pay tax thus won't know how quickly these details are checked).
