Wow. Thanks for all the thought and time taken to post replies!
I looked at the waveform from the inverter on a 'scope and found it to
be nearer 130V and around 5kHz which wasn not as it was specified to
me. The waveform appeared not too far off sinusoidal which I was
surprised at. I also found some MOC 3020s on Ebay and paid just 18
pence each for them so I'll give them a go anyway.
Let me know if you want a couple of them to play with and I'll stick
'em in the post for free.
Thanks again,

UK Pete