It doesn't look like anything has contacted the cylinder wall that shouldn't have.
Thanks for the pointers... I'm back at University now so wont be able to look at it again until next week, meanwhile I'll read up on it. Even if I can only get it to work for a few months that should be long enough for a good one to come up on eBay. I wouldn't mind a bigger (or quieter) compressor, although that will probably cost more than the £25 this one did at my local car boot sale!