George and David and anyone else who is interested!!
I think there are two problems here. They are not directly related to each other, but have both occurred at the same time.
The electronic shutter won't allow the flash to work. I was only trying that because the mechanical flash was firing too late. If the trigger (one of these:-
http://www.phototrigger.co.uk/) can't get the shutter to move quickly/early enough then either the trigger is at fault, or the Fuji is at fault or there is some basic incompatibility between them. The trigger works just fine with my Canon 5D3 so by default the trigger must be working correctly.
The way the trigger works is that I press a button that starts the solenoid that releases the drops. This also fires the camera. The camera fires the flash trigers - Yn622 set up - and captures the action.
I can see by eye that the flash is going off after the drops have performed and can hear the shutter going off too late. I can reduce the delay between the drops dropping and the shutter activiating, but even at minimum the shutter still fires too late.
This leads me to think that there is a delay in the shutter activation. I'm somewhat naively assuming that this is always present and is built in to the Fuji system, but can't be seen on "ordinary" photography where milliseconds don't matter.
My triggers/flash units do have PC connectors so I could have a looksee at that, but I think the major problem is the delay in the shutter activating.
David's idea of having the shutter open longer and using the flash to freeze the action is a possibility; that's the way I use a separate triger to capture bursting balloons etc, but that then needs me to work in a dark room so that only the action illuminated by the flash is captured. A long exposure will capture too much ambient light.
An interesting challenge.
(Time for Dad's Army - the highlight of Saturday night TV!!!!!!)