Does anyone know a contact phone number for Booking.com in the UK?

We have a cottage in Norfolk with three bedrooms and that is our only property available to let our for holidays on our vineyard. 

We have had more than one guest contact us asking if they can stay at the Vine House when we are fully booked - the listing apparently indicates an additional property, and shows availability when we do not have it.

I am concerned we will get a double booking unless booking.com help us out.

How can I resolve this problem? A quick phone call seems best to me but to what number?

I am thinking of leaving Booking.com as I am disappointed in the lack of telephone support. It is a fundamental need in a customer facing business.

Kind regards