@Alex532, Wales!! I did a lot of cycling in Wales back in the day....would love to see your place!
RE your question, I would not refund them. You asked them to tell you when they were arriving (I hope this was via the Airbnb messaging service!) and they did not do so, so you had no idea when they arrived and were thus unable to meet them. Not your fault!
However, I would not be surprised if they take out a dispute with Airbnb and Air sides with them - not because they are in the right, but in general they go with the guest (at least here in the US). You might preemptively offer a partial refund, or you can just wait and see. But they didn't do the minimum to let you know when they were arriving so that you can meet them.
In the future, you might set out lighted signs to guide them to the right place (especially as it gets dark earlier in the fall). I'm about to do this so that they are guided right to the correct door, where I have a key lockbox, so they can let themselves in when they arrive at all hours.