Age nor mileags is relevant.
Did they give you anything in writing which you signed to back up there statement about the mileage limitation? It should be fit for the purpose it was sold for.
It's down to them but you might have a fight on your hands but you should get all your costs back if you pursue it. You will need to demonstrate that they have been made aware of the problem and given the oppurtunity to rectify it.

Failing that buy some matches and have a bbq.

http://www.tradingstandards.gov.uk/a...icles-sum6.cfm

Edited to add that if it's driveable, take it back, hand them the keys and walk away and start proceedings to get your money back