well if you're insuring in your mums name why bother declaring at all, your insurance is invalid anyway as you're not the main driver... just dont come to any events and drive near me, thx 
if u want to modify your car admiral give good qoutes, and also if you was to have a crash even just as a named driver your insurance is not invalid is it?? btw admiral would only replace your car with a standard car if you was to write it off and not your modified one.
I`m with tesco and twice they blame me for two men who hit my car and knock of my wing mirror and if u car you lose your no claim bouns TESCO to me sucks they rubbish they stick up for muppets who hit my car and then blame me....
Yes it is valid other wise whats the point in adding named drivers?!. They would have to prove beyond reasonable doubt that you drove the car more than the 'Main Driver / Policy Holder' for them to invalidate insurance.
Yup def, some people just think that because your insuring it through a parent then you are driving it 24/7. While this isnt always the case, after all who's gona insure there son or daughter to learn to drive on a 2l turbo, or use an impreza to do the school run, many people do use the Punto etc as a second 'small' car, my mum did and it was totally standard.
I would imagine the insurers would ask why your mum wants to put sports suspension on her car, or 17" alloys etc etc, thats when they really get suspect about who's car it is!