Update to my last post.
I thought it was the mains filter causing the problem - turns out it was poor connection between the mains cable coming in and the filter. It took a LOT of checking of the mains cable, the wiring to the filter and the wiring from filter to the main board to narrow it down. There was sufficient connection between the spade clips that connect the mains cable to the mains filter to allow the machine to beep on power up BUT not enough to actually turn on. Talk about frustrating to find. I reclamped the spade clips and all is well now. If the problem returns I'll replace the clips with something a damn sight more secure.