Pamwe, the No 4 is a very common rifle in SA (as you'll know, it was the backbone of the SADF till the late 1960s and thereafter continued in Commando (militia) and SAP units till the 1990s). The various-length bolt heads must be present in abundance. I suggest get a good local gunsmith to sort it out for you. King's seem to be still functioning (in business since before the Anglo Zulu War) at 53 Hunter St, Durban; but there are bound to be others.
I should say, though, that I have (and have had) several S African .303s - none have had headspace problems - I wonder if headspace is the problem - what have you noticed wrong with the rifle?