My new USB leads have come and I still can't TX FT8 with the IC-705. This is most probably finger trouble. It is just possible it is a rig issue. When less tired I shall investigate. CAT control seems fine and RX.
UPDATE 1604z: I am still struggling! I have had two offers of help, but so far no luck. One person has offered help over Zoom, and I may have to avail myself of his help if all else fails. I have reloaded the drivers, swapped leads to make sure they are carrying data, rechecked settings in the IC-705 and WSJT-X. At the moment, I just cannot understand why it does not work. RX works fine. Darn my muddled brain! Years ago this would have been so easy to fix, or at least understand why it is not working.
UPDATE 2045z: Solved!! My problem was RF getting back into the rig. When I tried 2m on the external antenna all was well. It seems that 10m on the internal loop is not acceptable. Now I know what the problem was, I can try chokes on the USB lead. It seemed odd that despite everything I was doing seeming OK things still would not function. First 2m QSO on FT8. Now QRT.