Got the bastard!
Turns out the problem was due to our domain being renamed almost two years ago, still some hangers-on referencing the old domain.
In C:\Windows, in the SIMS.ini file:
"SIMSSetupsDirectory=\\\dfs\SIMS\Setups"
was still set to look at our old domain, thus thwarting the whole upgrade procedure.
Many thanks to the posters who've posted SIMS technical guides in various threads here, wouldn't have found out about this without them!