Thanks everyone for your responses. I see that some of you have tracked down a couple of other ways I can contact AirBnB. I will try those.
AirBnB are fine provided everything is running smoothly but if there is the slightest glitsch it seems to go pear-shaped. There is very little option on the site itself for you to contact them directly. And really, their suggestion to "ask the community" is asking other people to do their job for them. I found phone numbers by Googling "AirBnB" phone numbers online. Using these I did actually get to speak to a (shock) real human being! However, the human being was purely a frontline presence and was not empowered to pass me on to anyone with the knowledge or authority to tell me what on earth is happening. I have even emailed their CEO Brian Chesky (a billionaire) but no response of course. It's all enormously frustrating.
It has been suggested to me to put a tweet on their Twitter feed. I get paid via Paypal so it has also been suggested that I try to get Paypal to get the money from them. Haven't done this yet as it's all so timeconsuming.
AirBnB are very quick to pick up on requests trying to bypass their system (eg asking you to reply to a personal email address). They say this is because of security but it does also mean they would miss out on the commission for such bookings.
I haven't had any other trouble with them until now and my guests have been lovely. I set up a Paypal account quite easily and they pay the money (Sterling) into that and I then transfer it to my bank account.
Even IKEA offer better customer service – and that's saying something!