If I've seen a nice vacant block of land which I'd like to buy, in this case one in a nice suburb, what is the best way to go about it? Who has the records on the land ownership etc etc? Are they publically available?

The block of land in question has an overgrown half-built place on it and doesn't look like it has been touched for years.

Here in Chiang Mai, you check at the local land office. They have a copy of all land titles and should be able to tell you who the owner is. If in a district outside town you check at that particular district's land office. BKK may have more than one office I guess.