Welcome Smudge.
Yes, I'd start with the patch. You should only need the HOW patch if you installed both IWD and HOW. Other measures get a bit more drastic . . .
1) reinstall
2) delete the contents of the cache file
And measures bordering on cataclysmic include . . .
1) deleting the override folder
Others have had similar problems. But try the patch and see where that gets you. You also might try an old save game and replay for a while.
Good luck
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Most Subtle Official Straightman of the Laughing Hyenas -- Grammarian Brigade.