I travel around South Wales all week (skip driver) and the difference in accents is huge.
I work from Brynmawr, and 8 miles north on the A465 is Abergavenny who sound quite posh, and 8 miles south on the A465 is Merthyr who sound as Welshy (not sure that's a word) as you can get. Then there's the difference in Cardiff and Newport accents who border each other.
Is there a similar accent change in Cardiff? ie can a Splotty tell a Thornhill by their accent?