I bought mine from two wests and elliot onine. Horticultural suppliers. They have a few, but i purchased the simplest one. Can't remember price, but i am sure if you looked on web or rang them they would help. Good service too.
I can't imagine that they differ much, perhaps maybe in the amount they open. Don't think that you can limit the amount they open as that would defeat object.

One point is if your dad is tall, it don't half 'urt when you walk into or stand up underneath it!

Nick

Thank you for your reply....and advice about head hazards!! Sounds painful.
Any advice on whether to go for the wax or solar powered or does it make no difference?
Many Thanks
Nicky

I think mine is wax. Hotter it is the more it opens, quite simple and i can imagine it can go wrong. Never knew there was a slaor powered version... They just replace the existing handle thats positioned where you want it to stay open. will have a look

Nick