I've finally got it beaten!
I have eventually used four thumb joysticks of various sources - non of which centred 'off' if you see what I mean.... I got round it by (one channel at a time) prying the potentiometer off the side of the joystick, cutting off all the locating bits, and refixing it with globs of Milliput, rotating it till it was correctly zeroed, so nothing was happening. Only when it had completely set did I go on to the next one.... this took days......
A real bodge, but it actually worked, and I have a usable transmitter.
So I'm happy. I did have an accident early on in the process, where I had a couple of the crane ones done, turned it all on, but it lost the signal, and the winches happily wound everything up till things started breaking...... but I've got everything repaired again now. Job done.