Thanks for all the advice. I got a walk in PSA test for a total of 950 baht at Wellmed near Asoke, as recommended here. Quick, painless and my results emailed next day. Can't complain at all although I would say the service wasn't really over friendly, just business like along with about 5 staff chatting and laughing in a group at reception, Thai style. I'll go again when necessary. My reading was normal BTW.