David WE Roberts wrote:

One issue - in the summer it is going to be an oven.

Yup it sure is!

So I would like to have an opening window in the roof, to allow the
heat to escape during the summer days; much like a vent in a
greenhouse, or one of the opening roof lights in loft conversions or
single storey pitched roof extensions (or even conservatories).

My Wickes off the shelf conservatory has exactly that, so they might sell
them.

Polycarb does make our conservatory very hot in summer, I'd suggest blinds
as well if you are going to spend much time in there.

Dave