I ended up upgrading to Ji-Net's "Clean Cafe" service which should be 1024 download, 512 upload. We're paying 2000 baht a month which is cheaper than the list price because we were threatening to leave. I've just done some speed tests on
Speedtest.net - The Global Broadband Speed Test at 11:30 at night:
Bangkok 853/412
London 425/330
Los Angeles 618/425
Frankfurt 871/346
I'm pretty happy with the service so far.
By the way we're on Soi Nanai 6 (Soi Thumdee), up the hill a bit at the back of Patong.