I thought I better check the gearbox oil as I handn't done so and the gearbox does "sing" a little in top. Given the car's unknown history I decided an oil change was probably in order. I didn't measure what came out (as I dropped it into a container that already had some old engine oil in it). However it was a goodly amount of what looked and smelt like pretty clean gear oil. There was a bit of swarf on the magnetic drain plug but nothing too alarming. The factory manual I have states; 2 liters of SAE 80 gear oil for the 4 speed box.
I ended up purchasing Nulon "Smooth Shift" SAE 80, which is suitable for gearboxes and transaxles with both bevel and hypoid final drives. I also bought a tube of Nulon G70 oil treatment (which I'm pretty sure I read on here somewhere) that a forum member swears by it.
End result is quite a noticeable drop in transaxle noise, as to the "Smooth Shift" the car shifted smoothly anyway, so

.
My car doesn't have any under bonnet insulation and one fresh air inlet is missing the lower hose, so there are a couple of avenues for noise to become more apparent in the cabin than it should be. Anyway the noise that is present is a pleasant mechanical sound, not a death scream.
Rob.