So, I eventually defeated the wrathe, and then went to burn the letter in the little fire....

The screen went blank....

Loading appeared in the corner, and the serpent started to chase it's own tail.....

Around and around it went.....

Around some more.....

And more.....

and more...
...
..
.

So I though, hay ho, close, re-load, defeat her again (I had saved just before the fight) and try again.

Same result.

Restart my PC and try again.

Same result.

Alright, I'll try the More > Manage > Verify / Repair option

Load again, win again....

Same result.


Any ideas anyone?

OK, so it seems that the error here was that it was telling me that I needed to burn the letter still (text under mini map) but I didn't need to, so when I tried to it got confused. I just needed to go to get my reward.