Hi Mike,
A bit different from our experience, though same outcome.
We were towing from Wincanton to Chirk in early June, just beyond Shrewsbury we got the same "message" as you, and just before it appeared the car seemed to "hesitate" briefly. OH cancelled the message as all seemed well, plus there was absolutely nowhere to stop right then.
We continued a couple of miles then the autobox went crazy, slipping in and out of gear, engine revving wildly, the message reappeared along with the "stop immediately" red light. Luckily just around the next bend we found a lay-by!
Green Flag sent a transporter to take the car and caravan to the CC site at Chirk where we had a booking, and next day took us and the car to the Volvo dealer in Shrewsbury.
They hooked it up to the computer, tried a software upgrade and thought they had fixed it, but then took it for a test drive. Just as they pulled back into the workshop it went crazy again. Brief conference with Volvo HQ and a new gearbox was ordered. Meantime we were given an S80 to use, though it was not suitable to tow our van, so we had to cancel our next 2 sites and stay in Chirk. We got to know the area well as it took 18 days to get and fit the new gearbox.
Our fault codes also indicated a major failure, the mechanic reckoned it was most likely something electrical within the gearbox. Like you, we never did find out exactly what the problem was either.