Vauxhall Combo i have steam from filler cap when removed it sound like tractor and puffs of steam have no idea

Sounds ominous , at least back pressure due to engine wear . Check for coolant loss also but combined with engine knock really needs checked out .

No coolant loss been told could be stem valves or piston rings worried about price to fix as only paid for head gasket replacement few months ago

Check your engine breather is not carboned up ,causing back pressure ,and cylinder leakage test should show up where the wear is .depending on mileage,should really have had stem seals when gasket was done ,decoke of valves and pressure test of the cylinder head.

Whoever did the head gasket should have checked the valve stem oil seals . Could be cracked head also if it was not pressure tested and checked / skimmed . Other causes like problems with injectors /diesel knock but really hard to advise without being there .

Was replying when received a phone call and delayed post , hence similar reply to Bustedknuckles

Just been told could be injector seals due to the pressure the vapour is being lidded out

Does it smell like fuel ,the gasses emitted,would be the question? Not just steam then.

There’s no obvious smell of anything

You need to do the checks suggested before moving forward, cylinder leakage and check breather first