As promised: an update to what happened if anyone is reading this in the future.
The garage I used Changed the valves and seals at the top of the gearbox, this did not work unfortunately.
I then found an identical forklift for sale on ebay but in significantly worse condition - but the gearbox was absolutely fine!
The gearbox was swapped and it is now fixed.
It wasn't an easy job and they had a bad time getting the box to line up again but all is well.
It was a significant cost and I don't know if I would go down that road again.
But the guys at JLB specialist mechanics in Halifax are a decent, hardworking, honest and down to earth bunch. And I will always trust them with any jobs like this. They've done a great job.

I will now try to strip down my knackered gearbox and try to find new plates etc for it.

Cheers for reading.