Those who have been following the saga of getting everything working only to have the inverter fail may be pleased to know that its back up and running again. Two problems were sorted out - a pair of MOSFETs in the output H bridge were short circuit and the crimps on the internal connectors were done all wrong during manufacture.
The original eight 500volt 14amp irfp450 MOSFETs were replaced by STY34NB50 which are rated at 500volt 34amp. Higher drive requirements (twice the gate/source capacitance) but much lower ON resistance so should run cooler. So far so good!!
The internal connector lugs were all recrimped and soldered with a blowtorch and heatshrink sleeving applied where some of the cable insulation shrank back.
Very interesting having a good look inside - there are two main PCBs, one with 1kw of DC-DC converters (4 sections) and the H-bridge MOSFET output stage and the other main PCB with 8 DC-DC converters for the other 2kw of capability. A display PCB for one end with V & A bar LEDs and a fan at the other end and thats it!! The PCBs fused on each DC-DC converter with labeling that indicates that the same PCB is used for both 12 and 24 volt variants of the inverter.