I have used Cimple solutions (www.cimplesolutions.co.uk), located in Acton, for a Yamaha repair, but I think they do Fostex as well.
There was an ex-Fos engineer working out of a house in Burnt Oak (near Edgware) who did good work, but that was years ago. Maybe Fostex UK importer, SCV London could help: www.scvlondon.co.uk to find him or someone else?