Some progress to report...
I swapped over the TH3122 and popped it back into my tidied rat's nest. I noticed with relief that the Status line was low. I also saw it toggle and Dog saw that it was good. A few minutes later I noticed it go and stay high. Dog was less chuffed at this point.
The Th3122 was very hot and I now have another battle scar on the end of my testing finger. I interrupted the power feed out and put a 200mA DMM in line. Very, very peculiar, no current whatever. Short out the meter and up came the Nano. Next trick was to go to the Amps range where I read 198mA with it idle and ports disconnected. Dog decided he might be on to something here and swapped out the Nano. Peace once again reigned in the play room (temperature wise anyway).
I then uploaded my amended E46 sketch and still the TH3122 kept its cool. I set everything back up again and noticed activity on the LEDs that I had used. Quick look at Navcoder and after each status request was discarded data. Once again Dog was chuffed. The data was perfect except that the checksum was missing, so hopefully that will not be too difficult to find. Probably the array length.
Tx/Rx does have to be crossed on the Nano. Tx is pin one and Rx is pin two. You have them the other way about on your diagram for the Uno, Ian.
Letting the brain cool a bit now before I attack it with renewed vigour!