Thank you both for your comprehensive replies.
RE: having the gearbox reconditioned, I had a gearbox service performed at my local Ford dealership after they advised that it should resolve the issues I was having initially. It didn’t so they said next step is a new or reconditioned gearbox. I found an independent garage to recondition the gearbox. I’ll be honest, I’m not sure what this entails.
While I’m relatively competent at general vehicle maintenance and minor repairs, gearboxes etc are well outside my comfort zone, so I accepted that I had to trust that it was the gearbox that needed replacing or refurbed.
I will definitely be looking into the electrical issues that you have both raised as potential causes though. Thank you so much. I will update on any progress.