Thanks all, good intel. I can see why it's causing the engineers so much hassle.
I'm no engineer (you can probably tell!), but if the fuel valve isn't stopping it draining back, isn't the problem on the engine side of it rather than in the tank? I'm trying to do things that will isolate where the problem is occurring.
Having said that, I've not correlated if the problem ties with fuel levels. One to try for sure.
I'm sure all the flexible hoses have been pressure checked. And yes, check valve between FCU and burner has been changed (I remember the bill!)