Hi Ian,
It has to be marrow fat peas, I always loved the dry marrow fat peas that my Nan used to soak with the large tablet (this seemed to be soaking for ages and then left under a low light for hours) they were the best mushy peas I ever tasted.
These days I just use the tinned ones or I do have a modern frozen version of the above, which is great because you can just use as much as you need, downside they take about an hour on a slow simmer.
If I go into a pub I have to ask about the mushy peas, the garden peas mushed up with mint is not on, I have always been allergic to garden peas, any form of processed peas I am OK, so we always tins of Marrow Fat or mushy peas in our cupboards.
Regards
Alan