Thanks Scarlet dragon, that's really interesting. I notice on the local Facebook chicken forums here people are very quick to 'diagnose' Marek's without much evidence.
I know you are not supposed to mix vaccinated and unvaccinated chickens because of the risk of the vaccinated birds being asymptomatic carriers of Marek's, however this is something I have done without problems for many years (always have a few hybrids and ex batts amongst my home hatched stock). If it is Mareks in these two, it has not affected any of the others so I assume they must have good natural immunity.