Look on the strip down the edge of the door and you'll see a screw thats level with the bottom of the lock barrel. Unscrew it, Put your key in the lock turn the key to the one o'clock position and the lock barrel will pull out.
Measure the overall length of the old lock and measure from the cam in the middle to the outer edge (both sides) and that'll give you what you need to buy a new one.
45/50 are the most common sized euro cylinders i used to use, shouldnt pay more than 20 quid for a new 'un from B&Q or screwfix etc
hth