Any advice much appreciated.
Everything you even needed to know about flashguns. Including different flash trigger circuit suggestions way down the document. Basically it is a two terminal interface what has to have a 'standoff' voltage rating => then the flashes hotshoe voltage. http://repairfaq.cis.upenn.edu/Misc/strbfaq.htm#strboaof
Thanks but I don't need a flashgun circuit, I simply need the optoisolator to act as a very fast relay, when D13 goes high the other side of the gate has to close fast and stay closed only as long as the pin stays high