Sounds like they are just trying to fob you off with a dumb excuse. I don't think they've got a leg to stand on. At the very least they should offer a complete rebuild. Giving them the chance to put it right.
Who supplied the gear sets? You or them?
How many days after you paid for it did it blow up?
do they offer a warrenty as standard?
My personal experience of these guys only got as far as ringing up for a quote. The guy was over priced for a general rebuild (the quote he gave me) and sounded like he didn't really have time to talk to me, so I went locally.