does anyone know if it's expensive to re-glaze a conservatory from single to double glazing.

Mine is old and only single glazed about 12ft by 8ft and ideally I want it double glazed.

Cheers

probably cheaoer to start again wouldnt it!

probably :)

Depends.
What's it made of?
Can you get the old glass out?
Is whatever it's made of in good condition?

If the answers to the last two are "yes", then approach a glazier for quotes for double glazed sealed panels, and fix 'em with putty or Decorator's filler.

If the answer to either is "no", think about demolishing it, re-using the base, and building a "Wickes" one in it's place.
I have one, it was a doddle, even a pleasure to build.

Alcazar