I'm getting this code from my foxwell 520 scanner for the transmission (6 speed ) on my twin turbo sd (35d) , under full acceleration its revving out to 5k and sitting there instead of changing up and acceleration runs out completely like I'm suck in that gear or the gearbox is slipping and not able to put the power down , if I lift of it will eventually change in a few seconds. It can run out of puff before 70 mph but sit at 5k revs despite my foot deep in the carpet

No nasty noises and when driving sedately it runs fine at the minute.
Car is on 173,000 miles now and never to my knowledge had a gearbox oil change certainly not in last 40k in my ownership. I had it out in sport mode today and it is changing gears fine that way after clearing the code . My go to mechanic is off sick and not likely to be back anytime soon if at all so not sure what to do given the cars age and low value now .
Just wondering if anybody else has had that code ? as I'm not getting much on a basic search so far.