So I will post any updates on this in case it helps others, but due to the nature of the problem, think I may be on my own with it.
Bike starts and runs fine from cold. I have noticed the fuel pump runs continuously with the ignition on before starting. Pump stops when the clutch is pulled in, but the constant running I don't think is normal.
When warmed up and the fault occurs, the fuel pump relay clicks on and off rapidly (4 or 5 times a second). I have bought another fuel pump relay and side stand switch (OK, from a breaker) but I think these are symptoms rather than a fault.