OK - thanks MM for your promptings. I tried again, and with negative lead on the earth braid, it starts at 4.95V, then drops away every time i turn the machine on (with nothing bar J101 and J102 plugged in) BUT when i put the negative lead on the -ve rail of the power driver board its a solid 5.03V..... so seems to me the dodgy earth braid was causing wacky readings with the DMM.

Thanks for you help.