OK, thanks for the advice, I took it and rang easy jet. After finaly getting through their 'customer service experience' I managed to speak to a human, who much to my surprised was not only happy to confirm that the missing middle name would not be an issue (I checked this with passport control as well and they also confirmed it was not a problem), he then also agreed to add my other mates first name to the booking free of charge

.
When I tried on-line they wanted to charge me £20 each way to change it plus an uplift of the difference in the fare between when I booked it in November and the cost of the flight yesterday. Total cost would have been £116 per change.
I really thought they would ask me to pay, as it was my mistake and it is clearly outlined in their t&c's, so full marks to them for being reasonable on this one.
Archibold is a happy bunny as he thought he was stopping at home!! Gives us a few more beer tokens for our trip