I am with Carole Nash who seem happy to accept a long list of Mods - well bolt on accessories to both my bikes. My wife is with ebike this time who also seem ok with a list of bolt on mods. Anecdotally it's Bennetts, or at least the insurance co behind Bennetts that has a problem with modifications - even treating stickers as modifications.
My son was with Bennetts a couple of years ago and the usual thing when he changed bike mid policy the premium rocketed to a crazy amount so he cancelled and went elsewhere. Apart from the exorbitant cancellation fees it took about 3 months to get the pitiful refund despite repeated calls, letters, emails and escalations. Never again with Bennetts after that.
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