Zafira C 2012 Automatic Gearbox slipping. Any ideas? (A20 DTH 165bhp)

Hello all!

It's great to be back in Vauxhall land, after owning a P reg Astra estate many years back. I'm hoping someone may be able to help me.

I've bought a nice Zafira C, 2012 Diesel A20 DTH 165 BHP 2.0 litre automatic. Its a top spec car, but the previous owner didn't care much for the bodywork in a couple of places, and got rid of it when the gearbox started failing. So I'm going to make it look good again, but before I spend any time or cash on that, I need to solve the gearbox problem.

Symptoms:
When in drive, the car slips badly, to the point where it might not move. It appears to slip less when you get some momentum. When you put it in reverse, it doesn't seem to slip at all.

We did have an issue where there was a hole in the radiator for cooling the oil. We replaced that and topped the oil back up, but other than being able to drive through the gears (whereas initially it was stuck in third), it hasn't solved the slipping problem.

I took the solenoid pack off the box and tested it electrically, and all appeared to be within tollerance.

I did suspect the torque converter, but having realised it doesn't seem to slip in reverse, whereas it slips very badly in first, I suspect that the torque converter is not likely to be at fault?

Any help much appreciated! I don't have a big budget, and I like to try and solve problems / keen to learn. I also have a friend with a proper car ramp so we can get the box off.

The plate on the box has sadly worn, so not all the numbers are clear. I can read:
TF-80SC AF40.
Then I think it says W1280 beneath. Serial number starts 12C36...374 (can't read between).

Open to any ideas! (Or if you know anyone with a second hand box for sale, I may be interested! It would be great to save the car!

Thanks!

Paul.