A salutary lesson was well and truly learned. 
I didn't check my camera before I set out and while I had remembered all the things that usually catch people out, like turning the silent shutter off (so the flash trigger works), I had not cleaned the flash contacts. Consequently, all sorts of intermittent failures occurred. What made it even more confusing was that I was not the only person present that had problems, which just added to the confusion.
Fortunately, I always take two cameras to a shoot and the second one worked.
Today I got out lots of the kit and several things didn't work, and it was all electrical contact issues...

I didn't check my camera before I set out and while I had remembered all the things that usually catch people out, like turning the silent shutter off (so the flash trigger works), I had not cleaned the flash contacts. Consequently, all sorts of intermittent failures occurred. What made it even more confusing was that I was not the only person present that had problems, which just added to the confusion.
Fortunately, I always take two cameras to a shoot and the second one worked.
Today I got out lots of the kit and several things didn't work, and it was all electrical contact issues...