Well I think the truth is this (though I've lost the service sheet so I'm not absolutely sure). It's not that they'd added two verses. It's that the New English Hymnal has left one out, and also that the photocopy of the words and melody that we had in our choir folders was incomplete and had the last verse left off, due presumably to the fact that the last verse was over the page in the hymn books.
The New English Hymnal supplies seven verses of vexilla regis. The old English Hymnal supplied the same seven. They are given in J.M. Neale's translation but the NEH has made one change to the translation ("The universal Lord is he who reigns and triumphs from the tree" has now replaced "Amidst the nations, God, saith he, hath reigned and triumphed from the tree" in verse 3).
But what is the eighth verse that is missing?
Eight verses are given in Frederick Brittain's Penguin Book of Latin Verse attributed to Venantius Fortunatus. You don't have to go far to find one that's missing in our hymnals: here is verse 2 of the original:
Confixa clavis viscera
Tendens manus, vestigia,
Hic immolata est hostia.
Roughly this means "His innards were pierced through with the nails, stretching out his hands, his feet, for the sake of redemption he here was sacrificed as victim."
However if we thought that was the missing verse we're going too fast, because the last two verses of the Latin text are also not in the translation. In fact what we get in the EH and the NEH is only five of eight verses written by Venantius Fortunatus (530-609), with two extra verses that form the doxology which were apparently added (in Latin) by someone else a bit later. You can find the whole of it (ten verses, including the two that are not by VF) here along with the translation (of selected verses) by Walter Kirkham Blount (d 1717), which Michael Martin there suggests is considered to be the best one ever done. As you'll see, the three unfamiliar verses don't appear in Blount's translation either. So what we got in Bury St Edmunds was a bit more of what Venantius F wrote. However, since I don't have the service sheet with me I can't tell you exactly which other verse we got (but probably one of the last two judging by where it came in the hymn).
Not something to complain about, it seems to me. The more the merrier, I say.