Thanks for all the help.
I play an Eclipse Large Yellow with a more or less middle of he road leadpipe. Sometimes I swap to a tighter one, depending on the gig. My mouthpiece is a GR Bergeron studio. Granted, I've had a few slow weeks and not been practicing as I should, but that bloody high F is a sod. It's always been difficult on whichever horn and I manage to play a good couple of notes above it more or less regularly.
I use first valve. Or try to. It feels as though the slot is very small and the air struggles to get through. I always try to play the high register stuff PP and relaxed when I'm practising. I can do the "Knevitt - Cimera" method, exercise 16, well above the high G that is written, at p -mf but try to play the frigging high F on its own and.....aaarrggghhhhhhhh 30% success rate.