The transmission being stuck in 1 gear is an indication that it has gone into limp mode.
If the transmission is going into limp mode then it should have stored a fault code - the first thing to do is get the codes read on a diagnostic tool.
Also the oil level should be checked - there is no dipstick installed from the factory so a special tool is required (a dipstick! These are available from tool suppliers or also on ebay).
Check the transmission for oil leaks - this transmission is prone to leaking from the electrical connector on the front of the transmission . If leaking from here then the electrical spacer & seals in the transmission will need to be replaced & the electrical connector cleaned with electrical cleaner.
A common fault inside the transmission is with the electrical plate - this usually shows up as a code for the transmission speed sensors. The valve chest needs to be removed for this.
Another possibility can be undervoltage fault codes stored – this is often down to a faulty battery. The battery should be checked with an electronic battery tester if undervoltage fault codes are present in the transmission ECU.
I would also recommend a transmission oil & filter change & inspect inside the sump pan for any signs of swarf indicating internal wear or damage to the transmission .
The problem with a 2nd hand gearbox is that you could have internal faults still causing the same symptoms - or it could also be a bad Transmission control unit.
Either way you should really have a check for fault codes carried out before carrying out any further work.
Best regards, Paul
Thanks for all that. We have been told that because of it's age the gearbox is mechanical rather than electrical, do they still have the same problem with fault codes?