I would like to thank everyone who helped with this problem. Its fixed, at least for now but unfortunately I cant explain what fixed it as I took apart and cleaned most of the critical parts (excluding the MU) but including blowing through the pipework with compressed air. Whatever it was has now gone. I suspect crud in the pipework as suggested by Steve as it favours the symptoms as everything else looked fine. The one gain in this short saga, the pump has never sounded so quiet.
Once again - thanks all.