And it's back!
So...
I de-versioned the route by opening in CM and manually editing the config file to build 4.3.
Reinstalled TRS19 (again).
Imported the route into TRS19 by using the "Import content folder" in CM(19)
Opened the route in TRS19. Yes it opened ok with no other changes apart from the build number.
Saved with a different name, then opened for edit.
In CM(22) Imported the new version, still would not open from Trainz+ main screen.
In CM(22) I tried the <right-click>/Open for editing/Edit asset in Surveyor and got a message "Unable to Load Required Assets, Forcing database repair" (Or something very similar)
The DB repair would run through but the "Starting Trainz" just sat there doing nothing. Clicking OK closed the whole TRS22 processes.
I assume this is what is happening when trying to start from the Main Game Screen, but just with the "Unable to Load..." message not displaying.
I enabled all the disabled assets, still the same...
Back in TRS19, I deleted the missing assets (about a dozen or so) from the route, saved as another new version and imported that back into TRS22+.
CM22 was now telling me that a pile, about 50 or so, "Installed from DLS" assets were now faulty. Deleted these and re-downloaded them (1st class ticket helps!)
Now it works....


So it seems in TRS22/Plus build 1268280 a route will not load if it has missing dependencies, but with no warning.
If I can find what these missing dependencies are I think I should be able to get the whole route back. I've not yet found if any of them are critical or not. Why they didn't save to my back-up cdp of non-dls dependencies, I don't know...
Many thanks to all that have helped, much appreciated.
Edit: There are now, once again, several (97 in fact) installed/packaged/built-in assets that are faulty or missing dependencies. I suspect the forced shutdown of Trainz when it has "hung" is affecting these assets and making them faulty.