If the R4 Comedy is the one with Lucy Beaumont (Mrs John Richardson) and Maureen Lipman, then the answer is 'Hull'.
We saw Lucy Beaumont at the fringe last year. The first 2 minutes of her set were about that specific Hull shibboleth.
A colleague of mine is originally from Leeds and he retains the Leeds equivalent of the 'urr nurr'. He used to bring a briefcase into work, which impressed people until he opened it up one day and proclaimed plaintively, 'Urr nurr, nurrr cuurrk'. (He only used the briefcase to transport sandwiches and a can of fizzy drink). The phrase has stuck.