is there a north/south linguistic divide in england?

<<A lot of northerners have [a] in words like 'letter' and 'comma'. Doug Pebb, what do you have in these words?>>

I myself have /@/ in those words.

I reside in the South of England and I always pronounce bath like "bahth". I don't recall anybody commenting on the way I pronounce bath but some people have remarked on the way I pronounce plastic. Rather like bath, I pronounce plastic "plah-stick". Some people have told me I should pronounce this with a shortened a (like "cat").