The fact you paid cash isn't your problem. It's not your issue whether he pays TAX or not on that job. However the fact he's come back around and looked at his job, confirms he originally did it, especially if you have a witness.
This is exactly why I wouldn't do cash jobs over a couple of hundred quid. When they backfire, they backfire big time.
If he doesn't come back (and I'd simply say on the phone you have a problem can he come check to make sure you're just not being over cautious - he should take it from there then) then you may be able to get the job checked by an independent tiler. Then do a report on it, then go through small claims. Give the guy a chance first though. Nobody needs hassle.
There are cases going on that we're following on here though and its not nice for either party.