Two bulbs were of one brand and two of another, so they probably were not of the same age, but may have taken a few knocks I suppose. They blew in their respective pairs. It is stabalised and I will definitely check the voltage. One bloke I spoke to said that if oldish bulbs have sat idle for a while (this head has been in storage for 18 plus months) they tend to blow soon after being used again. May not be true, but worth a thought (cant see the mechanism tho)
I printy mono only so no real colour balance issues.