I seem to have fixed it !
Replaced the memory switch on the door and immediately had the correct operation of the memory functions, but still no drivers seat forward and back movement with the engine running.
At this point, I thought that I was still going to need to use SDD to diagnose and possibly have to replace the seat module.
Then I thought, what if it has a stored fault code generated by the faulty memory switch ?
Connected my code reader and, sure enough, B1B88 was showing.
Cleared the code and all working fine since then.