Oops,fell foul of the rule "if the lifters are noisy check the exhaust, if they are still noisy check the exhaust, if they are still noisy check the exhaust again!!"
My plug wire brackets hide a part of the header manifold, took them off to prepare for rocker cover removal and found a pin hole leak straight up to the rocker cover flange.
So a relatively easy fix!! and now I know what to do when the rebuild comes around! thanks.