Gear issue sorted - the culprit was some badly made shifter cables (they were cheap ones from Aliexpress - you'd think I'd know better by now...)
The cable was too long and not allowing the selector to engage fully. I removed this locknut and wound the cable eyelet further down the shaft and this resolved the issue.
Whilst I would rather have the locknut in place as i'm sure there's a reason for its existence, the cables work fine without them and can't unscrew anyway as they're held on to the gearbox with a cotter pin.
I've found since buying them that these are actually a cheap copy of a K-tuned cable design so it would not surprise me if the quality control was lacking, or they were actually copied wrongly in the first place
A few small minor issues to knock off and then MOT time!! 🤪