I've heard of a ridiculous situation where someone had stuck a sticker on the outside of their car. Because it had been modified from factory spec, they didn't pay out.
Things like heated seats etc - if they were optional extras that were fitted at the factory, they don't need to be declared. If they were dealer fitted or otherwise, then they need to be declared.
You don't want to be in a situation where an insurer won't may out, leaving you with a damaged, unroadworthy car, the other party's damaged vehicle and their medical and legal costs. You could easily become bankrupt and lose everything. All because you wanted to save a few bob.
Has anyone ever taken pics of their mods to show their insurers (as proof)? I'm now tempted to do that just so there's no doubt as to what's fitted to the car.