It may indeed be one of those mythical tales, I've got a vague recollection of hearing it while on a guided tour of the city walls at Chester (which would kind of make sense from at least a geographical standpoint, it being on the Welsh/English border), but of course me 'seeming to recall something' doesn't prove anything.
I do know part of the version I heard has some basis in fact, evidently it was often up to the local clergyman in villages and towns to organise archery practice for anyone over a certain age once a week, so that may be where the 'Sunday' bit comes from.
I'm sure someone will enlighten us with a slightly believable wiki link at some point.
Chock