Sodium carbonate is sometimes added to the water used to reconstitute the peas so as to prevent chlorophyll breaking down and preserve a bright green colour. Always offered with fish and chips in the North.
Mushy Peas - probably cooked with sodium carbonate
Although peas cooked until fallen is almost certainly an ancient dish (see: Pease Pudding) there doesn't seem to be any record of the actual phrase 'mushy peas' before the TV series 'Last of The Summer Wine' in 1973 with the line written by Roy Clarke; "Clegg: We only left him last night. Stuffing his face with fish, chips and mushy peas." (OED)
'Marrowfat' simply means a mature pea, plump and suitable for drying, and is not in any way connected with the Japanese 'Maro' pea.
2015