Thanks guys for all the input.
I pulled the valve cover off again last night and had a closer look, buried way in the back the intake rocker arm on the last piston had backed the factory nut off and the rocker arm was no longer contacting either the valve stem or the pushrod... gonna get some new nuts for the rocker arms tonight and put them in, had to drive the truck to work today and there wasn't a single bit of smoke out the back. kinda intrigued cos there wasn't any rattling at all that would have alerted me to the slipped rocker arm.