Here's an update guys.
I know what is causing the problem but dont have access to fix it. The thread was started by somebody whose account was deleted - I dont know why, it may be that they hadnt visited the board for an age, or that they requested to be removed or similar. Anyway, it seems that the forum wont allow threads started by a non existant user to be listed as a sticky. This is a forum bug - I'm not sure when we will be able to fix it.