'Real' pie & mash liquor is made with the water that the eels have been stewed in. Pie & mash shops also serve stewed eels with mash and liquor. You can order pie & mash on line from one of the oldest and most famous Pie & Mash shops in London.
http://www.manze.co.uk/
Incidentally, connoisseurs will know it's 'Pie Mash', not 'Pie & Mash'.
I haven't eaten it for years - but I love it. Vinegar is a must!
I like Jellied Eels too - with plenty of vinegar and white pepper.