Thanks for all the replies and particularly the suggestions to try the Frsky set-up.
Ordered one Fri morning from Giant Shark, it arrived Sat morning (did not pay for next day or anything like that). Installed it on Sat. Took about an hour all in and only because I was deliberately taking it slowly so I did not do something stupid with a soldering iron. I reckon if you can install your motor/speedo yourself then you can install one of these especially as the Frsky thread tells you exactly which wires to use!
Ran it Sun in my 2WD at TORCH and zero problems. So impressed I've already ordered a second receiver for my 4WD and a set to convert my son's Gemini 2.
Still not sure what the problem with the Spektrum stuff was but it is solved now
Cheers
Rob