Stupid question really, but I ve just bought a house with a bathroom window which contains what appears to be half of some sort of ventilator stuck in it. I ve done a bit of googling and it looks like it may be a vent-o-matic.

http://nas/content/live/askmediy.wholesaleglasscompany.co.uk/acatalog/Ventilators-Vent-a-Matic-Vent-Axia.html

Now I want to replace the vent without breaking the window, but it s firmly stuck in there, and I cannot measure the exact aperture in order to ascertain which model I need to buy.

Can anyone tell me which model I need from the attached picture?

Sorry to post such a silly question.

Sam