Thank you for all the suggestions.
I was going to do this yesterday (find out why the FT-710 does not work with FT8), but had no time. Today is make or break time: I shall check the firmware version, try at a lower speed that 38400 (probably 4800) and if all else fails give MLS a ring for help!
In the end, it has to be something daft and I shall say, "of course"!
UPDATE 1709z: It would appear my firmware on the FT-710 is not up to date. I am not sure if doing the reset to factory settings some weeks ago did this or whether these firmware updates post date the rig being shipped from the factory and MLS? Anyway, clearly I should make sure the firmware is fully up-to-date before I try to get FT8 working. The rig talks about CAT-3 settings and elsewhere CAT-1. This has left my small brain confused, HI.