Well I spoke to the vet today, and she's been in touch with the labs over the past week. They are convinced it's NOT thrombocytopaenia, and are saying it's a one off.
Blooming merry go round. It's good though, because now the vet is convinced it IS this, and that it's not possible to be a one off - she's done a good bit of research and talking, so she's pretty well up on it.
The upshot it - he's to get more tests done, as she's not happy with the fact it's a one off. He's to stay on pred, and we'll see.
He's still ok, happy, eating well, drinking fine, just the usual.