---------"everything goes through press print and bingo nothing. Put in an A4 envelope and it centralises perfectly"-------------
Are you saying that the printer makes the usual noises but no printing appears on the envelopes?
Or are you saying that when the final print command is made, absolutely nothing happens?
Either way, I suspect the problem lies with Word rather than the printer.
I am assuming that when you are printing envelopes, you are setting it up through the Word dropdown [Envelopes and Lables] menu, and choosing which size of envelope to use?
Perhaps not, and since you say it will print A4 envelopes, it may be you are setting the address like a document to print A4, but feeding smaller paper into the printer, then the "printing has not happened" by the time the paper is through!
A simple way to sort that is to make your text appear "earlier" by reference to the rulers.