take the barrel out and replace it.
on the metal strip on the edge of the door there is a screw directly in line with the barrel, remove this and turn the key approx a quarter turn and pull the barrel out.
measure the whole length and distance from both edges to centre of the screw hole. there are many sizes.
for ex. whole length 100mm edges to hole 50mm/50mm
100mm 60mm/40mm
and so on.
google `euro cylinder` or the likes of screwfix stock them, they`re pretty cheap
levilewy, April 2010