Many thanks for the good pointers. I re-did everything from scratch again, with even a different SD card. I also deleted the dcts and did as you said putting the ttgo.bif and F/A inside the map folder. I also put "cleanflash" in the setconfig file of the Navcore when I first ran the TT with the maps for the first time (even though I had run cleanflash originally as well.
It worked - though which bit was important and did the trick I don't know. Maybe my experience might help someone - if it doesn't work re-do it all again and take extra care.
Replacement TT is now working just the same as the old one did.

As for the bootloader, it is unchanged and working just fine. I had updated it on my old TT but it was not necessary. One day I might move to a bigger SD card and then need to update the bootloader for that reason, but for now I just make sure my maps are under 2GB and with this TT only running from SD (it has no internal memory for maps) it is easy to have different maps on different SD cards.