How much is "The finish of the ever ready epson derby 1984" painting by Peter Williams worth?

It depends how much someone is prepared to pay for it. However, there are a number of pieces of his work available to purchase, so you could start by looking for a similar piece. Some of his go for a few hundred pounds BUT...you say you have 126/130, which to me says you have a limited edition PRINT rather that the original painting. Try looking on the link below and perhaps contacting the place itself. Hope this helps.