You have a bug seemingly specific to the UK version of the game.
Download the special UK patch
here. I'm not sure whether the UK patch requires any special installation order - e.g. if you have to deinstall HoW and TotL first. Better safe than sorry, I suppose, so the install order would be IWD, then the UK patch, then HoW, and finally TotL.
The patch won't actually work with a saved game unless it has a chance to reload the map where the problem occurs by re-referencing the now-patched game data. So, load a saved game prior to the little map with the cave entrances - the autosave just before leaving the Wyrm's Tooth map will suffice.