Finally found why the AC wouldn't switch on the fans or compressor clutch. A badly repaired wiring plug at the pressure switch (the wires were just twisted together and pushed into the plug) so it was barely making contact. Waggling the plug made the clutch engage and the radiator fans spin up.
Chopped off the plug and used a couple of spade connectors until i can get a new plug.

It was 31c (88f) in the van which dropped nearly 10 degrees in 10 minutes with the AC on. Not sure how cold the air coming from the vents is but it seemed pretty cold.
It might possibly still need a top-up as hadn't worked in quite a few years before we bought it.
We had a quick rain shower this afternoon at around 4pm. Only 10 minutes long so it didn't make a difference to how muggy it feels outside. Still in the high 20s celscius.