Looks like I need a new tow car

Jeep Grand Cherokee.

We have no problems towing with that

Totally agree Mark, and if you get a bigger heavier van in the future it will cope without swapping car.

Trouble is .... you then have to replace a big tow car with another big tow car

Been there .... doing that 😊

That's if your one of these that like to change every couple of years but don't have to with a GC. You won't want to change it. As they say "each to their own".

I tend to keep cars for a while  ..... I wasn't sure when I test drove a Grand Cherokee  🙄

I think as suggested get it repaired. 

Join the Volvo forum Click here.

Ask the same question. 

There are a couple of learned members on there, one is called Che5hired and another called Clan.

If they pop up and answer listen to what they suggest. 

They will say get the codes read because as soon as you got the dash message " Transmission Service Required", codes were stored in the TCM (transmission control module).

You will need a Volvo specialist to read them. That will be either a dealer or independent Volvo specialist. You may be able to get a forum member local to you who has the Volvo diagnostic program to read the codes for you. 

My bet is on the control valve body in the gearbox. There is a good chance that dirt from overused transmission oil has blocked one or more solenoids within the control valve. Note that the Aisin autobox in your car is used in a large portion of manufacturers cars when a real torque converter box is prescribed by the vehicle maker. It needs oil changes to keep performing well. 

Unusually Volvo claim it to be a sealed for life unit which does not need the oil changing but then if you read the manual deeply it states change every 50k in arduous driving conditions... Like towing maybe? 

If your car is diesel then also(I think & ignore me if it's not) you have the earlier VW derived indestructible 5 pot engine. Why change to a lesser capacity can only just tow it vehicle? 

So for my money, and as a fellow Volvo owner so yes I am biased, repair it. If it was to cost £2k to repair that's just short of £6 per day over 12 months or less if you keep it longer. 

It's not unusual for a manufacturer to claim their auto box is sealed for life, I know that both Vauxhall & Mercedes claimed the same thing but then later suggested a mileage at which an oil/ATF change might be a good idea.