Well heres my interpratation, I think it was ment in the way of idiot or muppet or along them lines I would not think for a second that he would mean it in an insultive manner.
As for the meaning as far as I'm aware there are only 2 one being a descriptive term for an intermate part of a womans anatomy and the other meaning idiot or muppet or the like.
As I said above I would say it is the latter meaning rather than the first, this is the way I use it towards people I know (mainly to apprentices when they make a mistake in a joky way) and to other forum members via MSN and on the box and they use it towards me in the same manner.
I can see why you may be offended, but I'm sure it wasn't used to insult you. Its all down to interpratation and as on the forum it is read and not heard so you don't know the tone it was ment, twat is just one of many words that can be taken in this way.