Tell them, they buy and fit a new set for you. If they dont fix it, then you will pay them for the tyres out of good will for giving it a bash.
Bet you it makes it handle correctly. Look, as Digsy says, we arent scientists, but we do know what works. We've been round long enough to see people with this problem and every one (yes EVERY SINGLE ONE) who changed the tyres reported it completely changed the car.