Managed to get the 4 replacement parts from 4 different suppliers including Anglo Parts in Belgium. The 1st/2nd selector, the 3rd/4th selector, the selector lever and the selector interlock – they were all very worn. The selector lever should be a good fit with no play when inside the selectors – but they had over 5mm play which was amplified to about 10 times at the gear stick end. This was causing difficulty in selecting gears and sometimes not able to select any gear.
The total cost for all the gearbox parts including gaskets and oil seals was under £100 hopefully money well spent.
Looking at how brittle the old gaskets were i am pretty sure that this is the first time the gear box has been taken apart – although judging by marks on and around the drive flange i think the rear oil seal had possibly been changed some time in the past.