Seem to have managed it now, I wonder if the source files i was starting with had problems, as although they looked good, they were not displaying properly in RWGPS in terms of elevation and cues - and these were the Cycling UK versions!
I found another almost identifical full route, with full description, split in 4 places to give me my 5 day routing, all 5 saved as .FIT and sent to Garmin with no issues. I also saved them as .TCX to Garmin connect, along with the stage by stage files from CUK which seem OK as long as not saved as .FIT or split. So if all goes tits-up en-route, I have the backup of blue-toothing to the EDGE unit as well as ultimately the hard-copy map-book.