OK, I've sort of got round it. I have told Galaxy NOT to update the game automatically and then using Galaxy I have knocked the game back to version 2.21 which has allowed me to load the save games. ;)

The change log for 2.25 https://forums.ashesofthesingularity.com/482836 does not mention anything about the patch invalidating save games, however I have seen several other people complain on Stardocks forums about this in the past (Nobody from stardock ever bothered to respond that I could see).

I'm assuming that all AOTS patches break save game compatibility but Stardock don't appear to care. Very annoying from a paying customer point of view as I was meant to be enjoying the game and not trying to figure out why the developers decided to screw me over. I can understand technically why this could happy but not warning customers about it and not letting them finish the save game they are on before blitzing their progress is not the way to handle it.

At least I can now work round their issues and patch the game when I have finished with the current save game before I move on to the next.

Yep, patches break savegames. Been like that for a loooooooong time, longer even than longcat can remember.
Maybe there's some truth to it, when people call this game Ashes of the Benchularity? It seems mainly used and mentioned for it's (useless) integrated benchmark.