AND THE ANSWER IS!
"I'm an idiot, I'm an idiot" or was that from The Grinch that Stole Christmas?
Here's what happened:
When I bound my RX to the Spectrum TX, my RX battery was low. It still lit the RX up and flashed - but slowly. I thought the RX was properly bound. I checked another RX and same thing happened.
I had one of those "oh no moments" and ran out to the workshop and swapped batteries.
Sure enough, when I re-bound the RX, the flashing light was much faster and stronger and then went solid.
Hooked up the TK20 to the power, speakers and motors and BINGO - that was the problem!
Now, I'm off to the office in a much better mood.
tjs