Ok, got the pump off today, a little too easily I think! I took off the stud nuts, and one came happily out with the stud attached. attempting to undo the locknut (LH thread) I could feel the pump moving, not just in and out, but very fractionally up and down. Further inspection showed that the studs were only finger tight, (though the locknuts WERE tight) so I'm wondering if this could be part of the problem? Gave the non return valve ball bearing a bit of a poke while I was in there, and that seems to be free and moving on it's spring. The pump looks OK (I think) I have included a photo. Another slight puzzle, I rotated the drive easily with my finger, then took off the end cover to inpect the cogs (fine), I didn't remove the screws, left them in the cover, so they'd go back in same holes, but once I'd tightened it up again, I couldn't spin the drive with my finger! I'm assuming once the drive is being driven by the crank it will turn easily, but maybe if the pump WAS loose, it will surely have worn the pump drive and crank drive.If the pump was loose, could this be causing the problem, but if so, how? Also removed union bolts from rocker boxes, apertures look the same size to me (see photo). Leaving it apart for the time being whilst back at drawing board! PS, should I loctite the pump studs?