I know there's plenty of shops happy to make and sell you a suit but wondered if anyone could recommend where to get alternations done in HH or Cha-am. Thanks.

Any of the Tailors shops will alter your suit as required.

Workshop on soi 80 a few doors down from the top - i think this is where a lot of the tailor shops get there work done

Seems to me the guys in the shops are salesmen, the tailors are in a sweat shop somewhere. Given the, ah, um, reluctance of Thai merchants to ever admit they did a crappy job, what options would one have if the altered suit came back and the pattern didn't match up at the seams and they say that's not their problem?