Update:
After reading this blog I found out that the pressure switch can be bypassed for diagnostics. I did just that with a jumper cable at the motherboard contacts and hey presto, the spa starts to heat and the lights are back to normal.
So it looks like there is an issue with the pressure switch. From what I can find, it looks like this is a integrated component. Does any one have an exploded diagram of the heater, or know of any way to service it. Failing that, is there likely any issue with leaving this bypass in place permanently?
I have also noticed that when cleaning the filters, there is a LOT of white stuff that comes out of the filter with the silver ions cartridge in it. Is this normal? At the moment this junk is all over my patio... I can only imagine what the inside of the heater must look like.
Cheers,
hopoffbaby