Update on this, it started playing up more often in both auto & manual modes, eventually to the point of not changing at all and 'transmission malfunction' message on dash.

Had a code in TCM for a gear control error, description advised replacement of gearshift actuators. (£600+)

Stripped the old unit and the pin that actuates the fore>aft gear selection is very tight in the housing, the pin runs through some plastic bushes, you could probably ream these out for a cheap fix...but be aware that when you take the motor off the gear falls out, and if you don't get it back in the right place the thing won't work anyway...

So fitted the new assembly today, carried out transmission learn on IDS.

Sorted!