If you were to stick a 2.0ltr turbo engine under the hood along with a Nitrous boost i would go as far to say you should have your license ripped up in front of you for not declaring it. However i put a bracket in my boot the other day, it had a generic non-mx5 battery in it that moved about all over the shop. I put this bracket in hey-presto! no more shakey battery… however to be very sychphantic my car is now “modified” I can unmodify it again within 5 minutes… but where is the line drawn?
I’ve always declared everything I’ve done to my cars, even down to changing the wing mirrors. It costs you nothing to e mail the changes to the Insurers and gives them less of a chance to squirm out from paying out in the event of a claim