@Dreadnatty - Thanks for the link, I'll check it out. Phone's been running ICS since I bought it about a month ago but the problem seems to have developed/got much worse in the last week or so.
From my admittedly limited knowledge it seems to be a conflict between the FB app and FB for HTC Sense. I've been playing around with settings today and in my account sync settings for the FB app I have disabled auto-syncing of FB contacts. Since I did that it hasn't de-linked contacts so fingers crossed...