Hi Sue ,a friend looked for one last year and ended up buying a new one as hardly any available as its a big 3mts awning, so quite rare. The ones that were for sale were around the £1200 mark for a 2-3 year old one, people know that Isabella's hold their money and they do tend to get it.
The annex is even rarer with the 300 size, these dont become available very often at all?
Maybe Ebay is the place to look if you dont mind travelling, one thing to look out for it although it maybe 2 years old could have been on a permanent pitch so erected all the time, could you tell? probably not.
Good luck in your search