I am not a motor insurance expert, but if an owner of a modified car were to be involved in an RTC and the insurer refused to pay out, I think they would have to show that the modifications made a material contribution to the cause or outcome of the accident.
But having said that, what reason could there be to not drop an email to the insurance company listing all the modifications from standard? If the answer is "They might increase the premium", then as far as I can see either the mods are indeed a material change that should have been notified, or you are with the wrong insurer.
When I told Headley about the change in engine, they didn't change the premium, and that was going from something 97bhp to 165.
