Thanks @Rooted, the car has history but not enough to confirm Haldex serviced that regularly. Last service was in November (I took it to Skoda dealer just after buying as I didn't trust the used dealer much based on what they hadn't done that they said they were going to...) but before that it had been a couple of years and 40k.
The crazy thing about this is that as soon as I picked it up, I told them there was a problem (I got stuck) and EVENTUALLY last month after several visits they agreed there was an issue and to sort it out. As they don't have their own workshop, they took it somewhere and had a reconditioned whole rear transfer box put in - it took them 3.5 weeks to get the car back to me.
As soon as I got it back, I tested the 4x4 and didn't work, looked underneath and found that the shiny reconditioned unit had soldering paste on it and a part of one of the 'fins' missing. Naturally, I took it back and told them what I thought and in fairness, it got escalated by their customer service team (quite rightly).
They have now taken it to another workshop, who called me yesterday, and whilst he clearly didn't know how the Haldex should operate, he was very open to listening to me and shocked when he looked underneath, as by now the driveshaft seals were leaking liquid and the main seal was breaking up. In short, a joke, and he went back to tell the selling dealership this. That was yesterday and I'm yet to hear back again which is why I wanted to be 100% sure that what I was saying was correct and the rear wheels should have drive in those conditions.
Thanks to you and @skomaz I now am - thanks for the link to the haldexrepairs.co.uk page btw, I came across this yesterday and sent it to the workshop. Unfortunately, this is all taking place in Spain where I live and so he was going to use Google translate so let's hope it did a good job!