Whatever you do dont get a cheap cylinder .. you are obviously concerned about your security.
UPVC doors can have as many locking points as you like but they are only as good as the cylinder.
Have a look at this link , it's quite scary... I carry twelve of these keys and only use 3-4 of them to open most doors I need to get beyond... and anyone can buy them..!
https://www.youtube.com/watch?v=hr23tpWX8lM
Do yourself a favour and think about this kind of cylinder ..
http://cgi.ebay.co.uk...L&hash=item1e5b97267b
Altogether more difficult to open and you cant get a copy unless you have the coded security card.