Just had Rob Gaskin across to have a look at the car - super big thanks. Test drive confirmed in both our view to be coming from the back of the car, not gearbox related. Droning noise comes in at 70mph, when on power or deceleration. When coasting or wth clutch in it goes away completely.
We suspect drive shafts / prop joints worn. This is causing vibration which is then travelling through into the car. Next job is to have another look at the UJs and do a bit of swapping around of the shafts.
Again, thanks Rob for taking the time to drop by. Appreciated. Gary