This is a tough one, but so long as you were genuinely unaware of the discrepency in the mileage I can't see what the garage can do as it is hard to 'prove' the mileage is higher than on the odometer.
Yes, the steering wheel and pedals may be smooth and shiny, but this could simply be incorrect driving habits !!
My best guess would be the garage may pursue you for difference in value between the indicated mileage and what they 'believe' to be the true mileage.
Let us know what happens !