Very unlikely imo, how can they prove that you have driven the car most of the time.
I've known people who were found out by the insurance company using PIs against them in the past – a PI on "retainer", a set of possible cars, send him out with a camera – series of daily photos over a couple of weeks would show who the main driver is. This is how they got caught out, the insurance company simply phoned up and said we're cancelling your policy and keeping your cash.
Of course if your mum has another car and goes to work daily, all they have to do is check with your works CCTV, or follow her to work.
Suggesting using a PI is unlikely, I would normally say true, but now knowing it happens I'd personally not risk it.
They have whole departments dedicated to anti-fraud, why do you think they'd not find out?
http://www.easier.com/view/Finance/Insurance/Car/article-144402.html
And, of course, if you're just a named driver and not the main driver you'll not be building up NCB.
Marko, if Quinn 'do a deal' they're in breach of FSA regulations.
How do you work that out? Have I missed something? They all price-match?