@Roisin18 If you are not able to spend the whole day at home waiting for an uncommunicative guest, send a follow-up message to the guest via Airbnb to state what time window you will be on site to conduct the check-in.
If this time window passes and the guest has still not arrived or responded, that would be the time to get Airbnb involved. The proper thing to do on their end would be to treat the matter as a guest cancellation, under the terms of your policy, but generally they allow 24 hours for the other party to respond.
One consideration that might be relevant, if the guests are arriving from overseas, is that their flights may have been affected by the (now suspended) strike at Heathrow. If the guests supplied any information about where they were travelling from today, you can try to track it.