very strange, adding a driver should bring the price down unless they have a bad record which you say she doesnt. i added my dad to mine last year and it lowered the price by about £200 with Bell and im paying about £510 now, try on this mate
http://www.moneysupermarket.com/motor/ you can stick in your added driver info and see wheres offering a lower price, worked for me when i found that Bell quote
