Hello Beheaded_Limpet,
As Midget Gems stated, this is a known bug. Check out
this site for the necessary UK patch.
Once you've downloaded the patch, install it, then go back to a saved game from
before you enter the small area with tunnels branching off (an autosave will do), and continue. I'm fairly sure the patched version isn't used until the buggy area is freshly loaded.