I found that the ones that were already fallen had largely been stolen by local wildlife, so I decided to pop one into my foraging bag and head home with it to see if I could convince it to open up. A couple of hours later on, still in the confines of my bag, it had opened up of its own accord.
The next time I gathered some I got a larger number that were still closed and when I got home, I put them on top of the oven whilst I was cooking, for about an hour. They all slowly opened so I'd bet they'd do the same within an airing cupboard or other warm environment.
This has taught me that when you gather beech nuts, you gather a lot. A LOT. They're pretty tiny once they're out of their shells and then are peeled:
Also, when you start to peel them, quite a number are empty, somewhere between a quarter and a half of what I'd collected didn't contain the actual nut part.
I'm pleased I only gathered a small number because it meant I got to taste them and make sure I liked them and wasn't faced with a bag full of something I really didn't enjoy.
I don't like them raw at all, but as is usually the case, once they've been roasted the flavour is quite different.
I'll add the 'recipe' at my cookery blog.