"Caradad, Thankyou for the advice. the tear is at the bottom, near the metal pull down bit. Any advice on how the things come to bits after removal?"
It depends upon the age and make of the blind. Some had a slot in the top roller, others were stuck on with double sided tape.
Once you have removed the blind from the 'van. carefully remove the hooks from the bottom rail. The net should just slide out. The roller can be removed by easing it to one side and popping out the plastic tensioners. Turn the net around and re-assemble.
O'Leary Motorhomes have loads of blinds and flyscreens in stock - if you can persuade them did find the right one for you.