I also had this type of secondary glazing in my stone cottage when I first moved in.
As I neither bought nor installed it I can't comment on its cost but it sure seemed to work very well.
As Mike rightly noted, storage in summer could be an issue.
On a very positive note it did stop my house from burning down!

Long story short...stack of timber against flat roofed back of house caught fire (unattended BBQ + farm dog trying to get last sausage = fire). The house had a lovely warm orange glow behind it when we got home....I found the timber blazing, the guttering gone, the roof melting/burning and the window directly in front of the fire was burning with the glass smashed and melting. The perspex sheet secondary glazing was shying away from the heat by ballooning inwards but still holding firmly on the magnetic strip. Obviously I was very lucky to control the fire in time, but the perspex stopping the fire entering the house certainly saved a far more serious situation.
So in essence I’m very fond of perspex secondary glazing!
Uncletig
PS Before I get health and safety tips it was a long time ago, I always now water the BBQ after use and never leave food out! I learnt my lesson.
