rear door cylinder/keyhole

while out today the kids went to fetch there football from under the dickie seat when they brought the key back they gave me the key with the cylinder was still attached. When i gave the door the once over i couldn't see what keeps it in the door handle? Am i right in thinking that with the hot weather could some sort of gunk melt loose ?? if so how can i fix asap because im off to cardiff in the morning cheers..