It was actually the show themselves that outed him as a fake. One of the paraphycologists fed him false information regarding a person that had lived at the location (it was a totally fictional character) and then Derek became "possessed" by the spirit of that fictional person.
I didn't even know he got the sack though I did hear about him being possessed by a fictional character.
I'm not sure if all paranormal activity that appears on Most Haunted is actually real. I kind of think some of it is but not all. End of the day, their job is to make the programme entertaining and if they don't get much paranormal activity at one location then I suppose they have to liven it up a bit with a few fake noises, images etc hear and there.
All faked i'm afraid and just for entertainment which is a shame, as it would be nice tom have a genuine show that investigates paranormal activity for real.
One of our CB'ers is involved in a team of paranormalists and they seem to be the real Mccoy, and their show is on Sky.