Could an expert skier tell me if my bindings should have opened please?

That's a tough call. As an intermediate skier, you should be able to twist your boot out of your bindings while standing on flat, firm ground. (And I don't mean easily, but you should be able to force it out by applying a little muscle). If you can't, then its likely set too high.

However, another factor that comes into play is the angle of the impact you took. Ski bindings are not guaranteed to open every time you need them - sometimes you'll just have a fluke accident where the binding doesn't open for a variety of reasons even though its set at the proper DIN setting.

The most important thing right now is to get your knee looked at and make sure you didn't do any long term damage. And if you did, make sure you get it fixed and give yourself sufficient recovery time. Then next year, have your bindings checked by a professional before you head back out on the slopes.

It would be nice to know what your bindings were actually set at, but it sounds like they should have come off. If you cannot kick/twist out of your bindings while standing then they are probably set too high for somebody at your level.

There is also a chart that is typically used to set DIN's on bindings. It takes into account foot size, skiing ability and weight. Any ski shop should be able to set them appropriately for you. This begs the question who set them before?

Yes...they should have opened