Alan wrote:

How about using the 4th core of the three core & earth to the existing
two-way switches - and letting X10 take care of the 2 way switching needs?
Then the cores will become live, switched live, neutral and earth...?
2 of the switches I wish to control are part of a 2 way switching
arrangement so this could be reasonably easy to do.

Yes, this one's good: this is a pretty common usage of 3+E when wiring
fans with a run-on timer, for example, and is much cleaner than
dribbling return currents to earth or to some other circuit's neutral.