Janey,
Well, we were wrong!!!!
I called Daddie, and he said "the done thing" is to verbally invite the vicar to the reception, but then it is also generally the done thing for the vicar to decline! I think that is because they do a lot of weddings and would be too much to go to all the receptions, especially when they don't know anyone. Its just a politeness thing.
I guess the vicar may attend a reception if he/she is a family friend.
Hope this helps,
Jx