Thanks Parksy.Judging by your username the nearest caravan breakers to you might be
http://www.iwestmidlands.co.uk/profile/493693/Tipton/Premier-Towing-Centre/
They keep a stock of caravan windows from caravans that they've broken up.
A more comprehensive list of caravan breakers can be found on
http://www.caravanningnow.co.uk/caravanning/breakers.htm
There's a firm that will make new plastic windows if you have the old one for them to use as a pattern even if it's broken but I've forgotten the name, sorry.