they are the standard boxes for the car (78 model) with a fairey o/d fitted. any wisdom would be much appreciated.
It could be the overdrive at fault. They are notorious for the thrust washer on the input shaft to wear, but on mine it was the overdrive which popped into neutral at the worst time.

If it is the transfer lever jumping out , then maybe is the intermediate shaft bushes.

You do realise that the transfer case is part of the gearbox which weighs 170KG from memory. The labour to remove and replace especially with a Fairey which also has to be taken off first will be expensive.

IMHO, you would be best to bite the bullet and have the whole lot looked at while out and also replace the clutch and rear main seal.

I did that when I first got my 77 20 years ago, and although it cost me $2600 then, it was then trouble free until the bloody Fairey I put on started playing up.
Regards Philip A