‘… potwallopers grazing marsh.’

This is a variant of ‘potwaller’, a curious and arcane English method of deciding who gets to vote in a borough.  According to the OED:

‘A male householder or lodger with his own separate fireplace on which a pot could be boiled, which qualified him to vote in a parliamentary election in some English boroughs before the 1832 Reform Act. This system of determining the electorate was one of the abuses criticized by electoral reformers in the early 19th century.’

I prefer the sound and implication of ‘potwalloper’, as if you could vote by beating the base of a pot with a spoon.

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 92