Well I've finally tracked down the cause.. Re soldered the connectors in the interior and re built the valve block, seemed to be fine for a week, then it rained and it went all messed up again, finally got a rear right sensor fault, pulled the plug from it and found it was corroded, gave it a quick clean and spray with some electrical cleaner and it's working fine again, I shall get a new harness for that corner. Thanks for all the suggestions and at least by rebuilding the valve block and soldering that connectors I've removed possible problem points from the mix