When changing Euro cylinders .. the locksmith needs 2 things to complete the job on the first visit. He needs cylinders the right length and secondly the right finish (Brass, chrome, satin etc).
It's very important that the new cylinders are the right length, otherwise they can be opened without a key in seconds.
I often get asked to change locks after a snatched handbag has occured. The owner usually loses keys and purse when this happpens. I change all the cylinders and deadlocks on the ground floor, however many there may be.
Euro locks take less than 1 minute to change and a reasonable cylinder costs in the region of £20-25 .. dont get the cheapest you can.
My fees are supported by the local council, so they wouldn't reflect what another locksmith will charge.
What I can tell you is if the Police have given me 3-4 jobs in a day...
( 3-4 houses to change all the locks in ) I need to pass any extra work on to an independent locksmith to get it done the same day. We would usually get charged £100 + for one Euro cylinder.. (and £160+ for a mortice lock).
It's a lot of money when as I say .. I can remove the old one and replace it with a new one in under a minute.
.. it is a must that you change the cylinders asap. Having attended 100's of burglaries I am well aware that they are happening right now.
If someone unauthorised has a key to your house, the best way to prevent them from using it and opening your door, is to fit your existing key in the lock and partially turn it cylinder.