Exactly the same over here Alan,the insurance companies I have heard and not sure how true it is,refused a young blokes claim because he had not informed them of the huge stereo he had put in his car.
Not sure about over there but having many discussions with insurance assessors and advisors over here,ANY change from standard(what was sold with the vehicle at the time of purchase,whether it be standard or option) is classified as a modification,so if I was to buy an SX and simply change the wheels from 15" steel(SX Standard) to the 16" SLX alloys,that to them is technically a modification and can if they wish to be pedantic void your insurance,every change I have made to Bruce and Cyril has been reported and recorded by the insurance company,even doing this I could still loose a claim on Cyril,as his wheels and tyres are not one recommended on the tyre chart on the door pillar,so many PPL make changes to their vehicles and don't report them to their insurance companies,then wonder why their claims are rejected.