I decided in the end to try a bigger battery. The one I bought 6 months ago was listed as the correct one on eurocarparts site - it is a 031 70Amp 540cca, I think this one was a bit too weedy.
So I bought the other one they had listed, a 664 105Amp 760cca. I tried this morning, probably the coldest its been so far, I gave it 2 lots of glow plug, and it started after about 20 seconds of cranking.
Cured - I hope! (was a tight fit in the battery box though!)
That first battery you had was too small, iirc the recommended cca is 590 minimum for a disco.
I got mine from a local farm machinery shop, small tractor battery 1000cca. Starts without using glow plugs every time and only £70