They were about £13 each. Goes on front of the frame in the interior mounted with velcro straps and has a zip in the middle to unzip and open/close the window. Keeps the flies and moths out thankfully.

Although Ironically, I found a small spider camped on my net catching the bugs who get stuck on it! Lazy bug-ger can't build his own web! But I left him be.

It's an absolute lifesaver- beforehand I'd hate opening the window due to the insects. My bedroom would get extremely warm. Now I can vent air out and not worry about critters.