Hi ...
I had a little dig around on the OFHS's Marriage Index for you, but without any success.
There are (male) SHELLARD marriages recorded in 1783 (and before) and 1827, but nothing between these two dates. There is also no Joseph SHELLARD recorded on the index.
Have you tried looking in Northamptonshire - the county boundary is only a short distance north of Banbury?
Pennie