Thank you folks.
geophil .... this appeared to have worked, however, I am now encountering "Path too complex" errors as I try to move forward in building onto my map based on the terminus of my route. There is only one spline on the map, not unlike my previous session where I had no issues using the "Map images along path" mode with map tile servers.
I followed your instructions, loading the last available .lgb file. It opened up the full map from where I left off. I then proceeded to load up my route, which linked to the maps exactly as I had left off. To create the next tile of maps along my route, I opened up the Map Tile Server dialogue, focusing in to the area on the preview map to where my project ended, and selected it as the co-ordinate and scale provider (right arrow click). I chose "map images along path" and immediately got a "Path too complex error". I reduced my spline by a few points, and continued to receive that error as i tried to map images along the path. I don't get the error if I just use "single map image", but I don't progress with this option.
Did I fail to do something properly? I can't move forward as long as I keep getting this error.