If the bike`s legally registered, and the tester reasonable, then surely you can explain that as a non-domestic bike it had to be modified and that the switch is now disabled.
Either that, or screw a big 'self tapper' into the switch to stop it being operated!
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There`s no cure for stupidity… but with the endless supply of stupid people around me, I must be close to finding it!
The Lizard tells it like it is!
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Vampire?