Thanks! Can you give me any more details about which is the friction
hinge?
Does Scewfix sell these things?

A handy site is:

https://www.upvc-hardware.co.uk/hing...w-hinges-155/?
gclid=CPjWpbTglJwCFWIB4wodeUudeg

Or is.gd'ed:

http://is.gd/28cOp

I've bought happily from ebay, but there's loads of sites on the interwebs
selling the same stuff.

How to get the right size? It's easy to get the length and width - But the
stack height might be more tricky. Of course, once you've removed the
broken one it's easy, but then you have to put the broken one back while
ordering the new one

Oh, the screw hole spacing may differ from the old to the new hinge - A
Dremel (or similar) with a small drill bit might be useful to have to hand.

Al.