It is possible there is a pressure issue with the pump. Did you check the line pressure?

When the gearbox is engaged, the pressure should rise to between 7.6 and 18.1 bar (110 and 263 PSI) in Drive and to between 9.7 and 24.1 bar (140 and 350 PSI) in Reverse.

 

I didn't check the pressure (I don't have the tools/knowledge to do that). Am I right in saying if the pressure is low the box will need to come out to check the pump and replace the pressure control valve?

 

If the pump is faulty, the gearbox must be removed to change it. The pressure can be checked with the gearbox in situ as the fittings for line and control pressures are on the sides.

 

Thanks, is there a guide somewhere on how to do this for an amateur like me?

I can see where the fittings are from the manual you linked to previously, and the pressure range, but I'm not sure what equipment I'd need and how to connect/test...

 

Here's one for thirty quid:

https://www.ebay.co.uk/itm/Automatic...QAAOSwGMFbcpGA

The correct thread connector must be fitted to the gearbox and then install the gauge. Probably best to have one of your mates handy to start/stop the engine.

 

You may be able to find the procedure on YouTube. For example: