Thanks for all the help -I am excempt but i feel as though i better go anyway as the people at the local Job centre didnt appear to know the rules on exemption when I contacted them to try and cancel it.
I haven't been invited to a WFI for a few years now. I did go to one or two I think, and the last time I spoke to a lady on the phone and told her I wasn't fit for work (I'd just passed my first ATOS medical so was very confused ). She was fine about it,and not heard since,even since moving house.