So...... there is a happy ending to the 4th gear problem. Sourced original parts for both the fixed and sliding pinions from Vale-Onslow in Birmingham (really nice and helpful gang) that was fitted by my engineering mentor down south who took pity on me as I had zero time being up to my armpits in the midst of trying to get a 5" steam loco rebuilt and its boiler tested & certified (success!). He also made a top hat to shim out the selector spring just in case, so it made sure it was giving a really positive force. Comparing old dogs to new (got to be a joke there somewhere) the ones on the old sliding pinion did seem a little worn (rounded), but not massively so. Interestingly there was 4 thou play at right angles to the shaft, which may have also been a contributing factor to the lack of engagement at speed. However, everything is running as it should now with 4th staying put, even when loaded whilst going up stiff gradient, and my comfortable 50 mph cruising speed is happily achieved without jumping out of gear. On the way through this saga I bought a pattern (modern) manufacture sliding pinion part number 90-1693 from Kidderminster Motorcycles (also a helpful gang). This is unused as I luckily found an original part, but it looks fine and if anybody wants to buy it half price, let me know.