Nope, tried again, that wasn't it. I'll keep pecking away until I find out what makes it work and let you know what strange new way I found to break something I went through every page of the FMC and there were no boxes showing data needed and no discons in the route, and my manifest.json is showing version 0.1.5. This is procedure I followed for the FMC if it helps :
Pos Init
lsk last position to set ird pos
route
enter origin and dest (LSZH/EBBR)
dep/arr
departures
select runway then sid (28 - VEBI1X no trans)
route
enter airways / waypoints to last waypoint
VEBIT T51 LASUN T14 LUMEL T10 TORPA DCT EDOPI DCT GIVOR DCT SORAL DCT IBERA N853 DIK Y37 BATTY
act / exec
dep/arr enter arrival if known (star baty6a trans flo appr ilsY25R)
exec
*** try to save flight plan here
enter flight number on route page (JF1234)
perf init
enter zfw in while tonnes (what you put in the efb)(30)
enter reserves and crs alt (0.9,26000/260)
enter crs wind (270/3) and Trans Alt
exec
*** try to save flight plan here
1st edit - to make sure we were on the same page about directories I created a new plan in simbrief and downloaded it, and moved the .RTE file to the directory mentioned in the FMC manual (fs2024 steam). Then I reloaded the RJ100 and tried loading that from the company route field - great, did exactly what it says on the tin. Exited back to the main menu. Then I created a different plan in simbrief, loaded up the RJ70 in case it was just an RJ100 problem, imported that flightplan from simbrief, and tried to save it. Poo, wasn't that. But when I looked in the saved flights, the one I downloaded as a .FLT file was there and available. I've run out of time for tonight but I'll keep picking away at it as time permi ..... perm ..... per ... back in a minute !
Bother. I had a thought it might be a permissions thing so I cut Steam out of the loop and directly launched the MSFS2024 exe file as an administrator. Still couldn't save anything. Really have run out of time now, will try again tomorrow.