Detent maybe? Yup!

It's taken a little longer than usual to sort this out due to CoVid restrictions, but I can confirm it was a broken detent spring, now replaced and gearbox is now operating as it should and my clutch is unharmed from the short ride. Will be out on the road soon, just awaiting a new gasket. Thanks all for your input.
 
Always a relief, detent spring and not a circlip. Diy job around £10 dealer job £300 plus.
 
If you are waiting on the gasket can you post pictures of the installed spring and especially the end on the shaft? Without the basket ? Thanks