I have a whine / high pitched noise, it is only audible at 22-26 mph, when not accelerating, but going steadily in gear

It sounds coming from 'up front,
Any ideas as to howi can further my investgation,?

The noise goes away when I depress the clutch pedal, so I geuss it must be gear box'y
Maybe the diff?

I like many have neglected changing the gearbox oil, but changed it with whichever brand viking supplies

Ir was changed say about 4000 miles ago so could do with a change soon

Its not bothering me, id just like to know what it is ......

more likley to be input or mainshaft rear

however they all do that

replacement of input means pulling the box apart - the rears there is no harm in taking the back cover off and tightening the nuts on the back of the shaft

the bottom always runs in oil, the diff is massive and well oiled so generally you can rule them out. Main culprits are top rear where it starts to run or the nut loosens allowing the race to become noisy, the input shaft is supported at the other end by the crankshaft pilot bearing, it also has a nylon ball cage which goes brittle with age and can break up - but youd know if that had happened.

oil level is correct? clutch pedal free play correct?

Clutch pedal correct

Oil level will be checked

Cheers Sean
So nothing to fuss about

I'll take the back cover off and do the tightening when I pull the engine out in a few weeks

You may try the 80W90 GL-4 transmission oil for quiter running.

Even banana soup will do