Unfortunately, I am one of the poor sods who talked Kol before going into Serpent Temple but was not given the heretic key. Seems like there is no way of getting this because (now that I sovled all the other temple puzzles) Kol is no where to be found.

I saw other posts on a possibility of editing your save game file with a hex editor to put back a lost/missing item. Anyone have any ideas on doing this with the heretic key? If not, anyone else have this same problem and solve it without such a complicated solution?

Any help much appreciated!

It's no big deal - all that's there is two or three ankhs, a necklace and a couple of el cheapo scrolls. However, later in the game (dragonspire) you'll get to talk to Kol again, so you may be able to get the key then.