Always meant to sit down and write this out.....
Firstly G6 on the clutch shaft is always engaged with G7 on the layshaft. The constant mesh pinions.
For first gear G9 engages the dogs of G10 which is turning driven by the G6/G7 constant mesh pinions. G9 also engages the big end of the double gear G11 as it slides on the output shaft spline driving the output shaft and final drive sprocket.
For second gear G8/1 engages the dogs of G7 and also the teeth of the small end of G11 as it slides on the output shaft spline driving the output shaft and final drive sprocket.
For third gear G10 still turning through the engagement of the constant mesh pinions, and is also in constant mesh with G12 the third gear pinion, now has the large end of the double gear slide over the driving dogs which like first and second above Drive the output shaft and final drive sprocket.
For fourth gear, often referred to as direct drive as it really doesn't involve any speed change through gears, is obtained by the internal small end of the double gear G11 engaging the external dogs of the first constant mesh pinion G6. This makes the out put shaft turn the same speed as the clutch shaft so the final drive sprocket turns the same RPM as the clutch shaft.
All other gears are not engaged in each of the above scenarios, and also in neutral, should any issue occur to engage more gears, say for instance G8 becoming welded to the layshaft at around 100 MPH, disaster can ensue, don't ask how I know this except to say that a very wiggly black stripe measuring 208 feet (not stretched out but as the crow flies) and a trip to the haberdasher are occurrences to be dealt with.
So to answer Len, G12 and G 11 are likely the culprits!