Thanks for the replies, I think my best bet is to load a save where I have all the items minus 1 then when I collect the last item hopefully it pops up.
I will give that a go, I'm not really into hacking for achievements but Having put 100 hours into this game on this playthrough and having played through the game many many times before, it will be a while before I get round to doing another run through just to get an achievement. That one missing achievement will really bother me.
Worst comes to worst I will just look for a save around the time of picking up Occult Fan III and play on from there.