I'm grateful to all of you who have responded to my original post, and, in the style of some other fora, this could now be ticked as 'answered'.
To answer Justanoldmuso briefly:
The EK-50 is in effect a bit of a loss-leader in the Korg range. It offers a large range of sounds, some better than others, but the best very good. It also offers very good control for live play. It is easy to split or not split, and there are three channels on the RH side of the split, or on the unsplit keyboard, where you can set three voices, with easy control of volume and pitch, and solo or layer them on a one-press basis. These set-ups can themselves be saved in sets of four so you can move from one to another again on a one-press basis.
It has a record facility (ie record as midi) which is fine on a single pass, but less good on successive passes. And it is somewhat inflexible, too, re what you can do by way of play-along with such recordings.
So in terms of assembling backing tracks to play live against, I think the more promising options are either to record straight to audio, or record to midi outside the keyboard, and then edit and recombine the elements so recorded. RealBand is a strong candidate for the midi-record stage here, but I would almost certainly do the editing in other software which I already know very well.

Thanks again to all who joined in.

Ian G
Wales UK