Team AFM in Martleshan seem to be good according to rtbiscuit, and then there's GBH Motorsport in Langham near Colchester (They have a Zed owner on their books) and then we go further south.
DMS in Erith, whom I have used myself before, great service and good price too (if i remember correctly, Dixon did me a P3 for £350-ish, all fluids included)