You don't need an SSR to trigger a flash, you just need an opto isolator. A mosfet changes its resistance from very high to very low, that is what you want to do. A flash gun has a high voltage on the switch contacts maybe 600V or more so it is important to get an opto with an output FET that can handle this sort of voltage. Don't be confused by the isolation voltage it is not the same thing as the breakdown voltage of the output.
You can get the AQW280EH from the UK but the postage might be high:-http://uk.farnell.com/panasonic-ew/aqw280eh/photomos-relay-1500v-120ma/dp/1853864?Ntt=AQW280EH
maybe you can get other stuff at the same time or search the site for something similar.
for a fairly hefty £2.64
That is a good price for something with that spec.
Ah great, thank you, I didn't even think to check Farnell, but their prices are pretty much the same as Mouser's. I think I'll see what kind of PhotoMOSs RS Components have in stock and try to find something similar to the AQW280EH maybe.
Do you mind me asking what the isolation voltage and breakdown voltage are? I'm guessing the isolation voltage isn't something I have to pay much attention to right now, given that the AQW280EH's is 5,000V. Is the breakdown voltage the same as the load voltage? My flashgun's trigger voltage is between 220-230V so the AQW280EH's load voltage of 350V should handle that fine.
Thanks again, Mike, you're a great help!