I removed one of the openers for the greenhouse vents. Amazingly the company that made it (Thermoforce) is still around, so I emailed to ask if they could help with spares.
Sadly they couldn’t. A very polite response came back to the effect that it’s very old and they just don’t make anything like it any more. The only suggestion they could offer was to replace it with two modern openers (because the vents are actually twice the size of the ones that tend to be on modern greenhouses). I’m not sure I like that idea though, as it means the vent may not open evenly.
So, we’re off to Bodge City then. I’m going to order a couple of standard vent opening cylinders and see what I can come up with.