Bob,
Many years of research (taking the piss really) have taught me not to bother with the "You're not from Manchester" line.
I tell them that Bury are proud to be from Bury, Rochdale celebrate their home of Rochdale. Oldham are chuffed to fuck to represent Oldham, Stockport likewise...."SO WHY ARE THE RAGS SO ASHAMED TO COME FROM TRAFFORD?"...What is so piss poor about Trafford that they daren't admit where they're from?
It shoves the onus on them to explain.....and remember the newton heath foundation point is garbage...Newton Heath only became part of Manchester in 1890...as did Clayton. They were founded outside Manchester too.