The boys bike has developed play in the headset area, vast amounts of grease and random tightening has not fixed it. I am thinking new bearings but don’t know what to buy and need them quickly. He has a trial session at a club in sat so need to get it sorted. The spec is GWH A-Headset and it has those type of bearings that are open rather than what I would class as a trad closed bearing. What do I need?

Just replace the whole headset with one that used sealed bearings.

Ok wiggles, sounds sensible but are all headsets equal with regards to size and fit? So if I just bought a mountain bike headset from crc would it fit? Sorry, I know it sounds stupid, I have built two bikes but work by knowing the exact parts required for those bikes. Googling GWH A-Headset is not yielding the same results as I would get for ‘headset for cotic rocket’

No you need to measure the head tube internal diameter (which means getting the old one out first) it is probably just a standard old style one but that is just a guess. I would advise getting a LBS to do it as they will have the proper tools to fit and remove and are likely to have the headset in stock.