I'm struggling to see why there would be a back charge. A charge for what exactly - the period of time that's already elapsed without any claims being presented?
It doesn't matter if a claim was presented, a product was purchased for 12 months and used under false pretences. The fact is it does work like this and always has, they can and will levy a charge.
Best you could hope for is a reduction as a good will gesture, wouldn't hold my breath though.
If there were no back charges i'd insure all my cars as a 1.3 1996 Ford KA, say I live in the lowest risk area possible and I do 50 miles a year then just pay the additional amount in the event I make a claim for that year.