External shade
 

I shade my greenhouses in the summer and I now searching for EXTERNAL blinds fitting cedar greenhouses. Here in the UK there are plenty which fit aluminium houses, and plenty of internal blinds, but nothing suitable for my needs. To reduce heat I need to stop the sun from getting through the glass.
Standard shade cloth is not appropriate for my needs.
Any suggestions?

I'm sure you've probably already considered it, but just in case it slipped your mind, have you thought about ShadePaint? It might do as a temp solution while you source what you really want!

I dont know about blinds specifically but there is a aluminum shadecloth type product that has been mentioned by a couple of orchid growers I follow on YouTube.

Aluminet.

I have used huge sheets of Aluminet for many years and it serves me well but the task of spreading over my glasshouses is getting difficult so I am looking for a simpler less physical solution - hence perhaps roller shade of some sort. Alternative might be to leave it on all year but I doubt whether our dull winters would send sufficient light through in the dark months.
It is not the shade material I am asking, but an easier method of installation/removal.

May not be quite as attractive, but perhaps rig supports for shadecloth that can be put up on the inside of the GH above the plants, perhaps just the more sensitive ones? I can totally relate to not wanting to climb ladders to get the shade on the outside... (none of us is getting any younger :( <sigh>)

The late Joe Kunische, of Bloomfield orchids, automated his shade cloth. It was like a motorized roll-up window shade. He once told me he was planning to fully automate it, using photocells in the greenhouse to detect the light intensity, but as he had a full-time assistant on hand, he merely had to say so, and it was done.

They have fully automated versions that retract when conditions are cloudy. I've seen it in action at a friend's greenhouse in Houston.