All, need some help I have a trickle vent in a upvc window that needs replacing Firstly how do I measure for a replacement , so I get one with the correct size. Im guessing its screw to screw, full length and depth? Also the 2nd picture shows the outside. Do i need to replace these aswell? They seem like 2 plug in vents to me If i do need to replace the external ones how do i get the following outside flap off to get to them. Its very sturdy. Thanks in advance!