Unfortunately mushy peas has been something that passed Edinburgh chippys by, well as far as my experience goes, which is quite extensive given my age. Ahem.
However I'm a big fan, especially as I don't like the texture of normal peas (that 'pop' - yuck). I saw Jamie Oliver make it with frozen peas, a small sliced potato and some tartare sauce so that's the version I have with my fish and chips.