I don't know the actual recipe, but pie & mash shops usually sell stewed eels as well, and I believe the liquor is made with the water the eels have been stewed in, and parsley.
Cheats way. IE all the pubs in london i ever worked used was
heat a spoon of butter in a pan add a bit of plain flour, and mix into a thick paste ( a rue)
add milk a little at a time till it reachs sauce thickness.
then crumble in 1 or 2 fish stock cube the ones with parsley in them, and stir over low heat till till mixed in well. dont add salt as there is plenty of salt in the stock cube.
hope you like it.