Right, to finalise all of this, I have split the top ball joint and the driveshaft slipped out of the pot joint with no trouble. Here was the problem. I brought new CV's, driveshafts (Minispares monte type), but in reading the description, they say that the large clip is obsolete as it is welded to the driveshafts. What I didn't realise, is that the small retaining clip does not come with the driveshafts, which is still required, and this allowed the centre of the CV joint to slide off the splines, further into the pot, meaning loss of drive. Simon from Minispares very kindly and helpfully is going to put 4 in the post this afternoon because there's a good chance there won't be any on the outer CV's either at that rate. Seems a shame £144 worth of driveshafts don't include something as inexpensive but important as this already on them.
Still, hopefully that'll be that problem sorted! Thanks again all!