Bizarre. Bearing in mind I have inspected/re-inspected the new valves, swapped back to the old ones, slushed the IC and AC and all other feed pipes through to the check valve, I took the check valve off, looked at it, could see no obstruction, blew some low pressure air through it with no obvious pollutant, reassembled and fired up...no problem. So far I have filled a few cylinders including a 200/10l in about 9 mins without issue. Big fill session in the morning so fingers crossed and thanks for help.