Garage Charging For Parts That Did Not Need Replacing

[kelfoan]

I took my car to a garage due to having a leak running in the boot. The garage said it was a seal on one of the air vents that was perished and letting in water. They charged me £90 for parts and labour. However the leak continued so I returned to the garage. They had another look and this time they used sealant as to try and stop water entering the boot. Again, this did not work. I therefore returned to the garage and amicably asked that either they gave me a refund or made further investigations into the cause of the leak. These conversations were all very amicable as both myself and the garage could not understand how the water was entering the boot. The garage said they would give me a refund as they had spent a lot of time trying to trace the leak without success and could not see what else they could do. . However I was told that they would only refund the labour and not the cost of the air vent they originally fitted. I said that this was not acceptable as clearly it was not the air vent causing the leak and so did not need replacing in the first place. Eventually the garage refunded the full £90 but said I was not welcome there again as they were out of pocket. I was totally shocked by this response as I thought myself and the garage and myself were getting along fine trying to sort the problem. My question is: are the garage legally allowed to deduct for the parts they used (as clearly they did not need replacing) . If someone could also advise what legislation covers this scenario. Thanks in advance