Thanks for all the comments guy. As you say, I'm just lucky it wasn't worse.
I'm glad that I managed to get out of the skid which was taking me towards the centre barrier, and I'm just glad that there weren't other cars in the lanes that I ended up going across. I'm also lucky that I had a relatively soft exit from the hard shoulder. My right rear wheel took a hell of a thud on the kerb at the edge of the hard shoulder but a barrier or drop at that side could've been a lot worse.
Whether or not winter tyres would've helped is up for debate I guess. I'm not sure myself, but in future, I'm going to go for them. For the sake of a comparitavely small amount of money, I'm willing to take the chance that they're at least marginally better than the summer ones. I may also consider the possibility of sticking the Z in the garage from Dec to Feb.
Once the skid got into full swing, I felt completely out of control and I'm not convinced that, after pulling left away from the barrier on the initial skid, there was much I could do.
nottyash: I've not had any proper skid control training, but my understanding was that depressing the clutch, staying off the breaks and steering gently out of the skid is the correct method. I thought that with the car in gear, the wheels are unlikely to roll freely and are less likely to regain traction? But as I say, I'm really no expert.
Thanks again guys. Stay safe!