HiWarm it up til it ticks again, use a screwdriver as a stethoscope tp pinpoint the cylinder . Pull the plug lead and see if the noise remains, if it goes look at the exhaust gasket on that cylinder. if it remains check valve gear on that pot.
PS Still waiting on some measurements from you .
As Kev said if it's not exhaust leak :beuj: I take it their Hyd tappets, if so why not try adjusting them while engine at very lowest idle possible, makes abit of a mess, but at least your find the bugger :tup: