Found this offer in the Luton & Dunstable Express newspaper.
Basically order any medium or large pizza and you'll get a 2nd for just £1.
This seems to be only over the phone and is not automatic when ordering so remember to quote this before ordering.
Offer valide on delivery and collection.
Luton town centre - 01582 45 11 55
Luton Sundon Park - 01582 50 50 55
Dunstable - 01582 66 14 44
I'm not sure if this is available elsewhere but maybe worth a call.