I had a good long session this weekend and think I've cracked it!
I tried all of the suggestions various people made and also changed the dilution (i.e. the %age of gelatine) but started with 100ml plain distilled water in which I dissolve just 10g of gekatine and nothing else. This coated much the same as my original dummy emulsion with the food colouring etc. added. This proved at least that the additives were not the cause of my difficulty.
I then went through all of the suggestions one at a time, adding to this first batch of gelatine. Rather than do everything at once, I wanted to see if there was a 'Eureka' moment. There wasn't, really.
I won't detail everything here as the post will be too long. In a nutshell:
Alcohol and photoflo helped a lot.
So did keeping the emulsion at 40 degrees.
Diluting the emulsion to 7.5% gelatine made the biggest improvement.
Plate cleanliness wasn't my problem.
Breathing on the plate and warming the plate to 40 degrees made the emulsion flow more easily, but also made it run off the edges much easier, too! I managed better without these steps (Room temp was about 25' C anyway).
I think my problem was my first dummy emulsion was too viscous. It flowed badly and set far too thick. The 7.5% version with the alcohol and photoflow at 40 degrees C flowed much better and I was able to coat a dozen plates very well and they set with a nice thin, hard and even coat. I'm sure the fact that I've coated about 50 plates now has helped, too.
Now, to see if I can repeat this with a real emulsion...