" I have always assumed that these doors were burglar proof"
Sorry to dishearten you dennis... but when I am asked to gain entry to a house that has a UPVC door, it brings a smile to my face because I know it wont take more than a couple of minutes. You can have as many locking points as you want (hooks, bolts and mushrooms) , no matter how many you have , they are only as good as the cylinder into which you put your key.
With a little bit or research you will find that if this type of door is closed only on the latch it can easily be opened with a plastic card... I use the lid off an ice cream carton.
As for 237SJ question... I have fitted hundreds of chains to UPVC doors. But you must realise that many doors only have a plastic skin of less than 2mm.
Screws will pull out of this if any pressure is put on them.
Better quality UPVC doors have an aluminium box section under the plastic, these give a more secure fixing.
As JJ said... a chain is just used when answering the door, it's not meant for added security at night.
Providing the door does not have a letter box in the middle of it, why not change the cylinder to a thumbturn cylinder(no key on the inside).
This means that she can engage the multipoint locking system at night by lifting the lever, and then turning the thumbturn cylinder.
The reason I mention the letter box is that a thumbturn can be opened in seconds through a letter box.
For anyone out there who has a UPVC door and is now worried about their security, remember the cylinder is the weak point on your door... and as Naomi previously said .. if they come prepared to get in..you wont stop them.
...Sleep tight...!