How to rectify a mistake of double booking made by booking.com on dates I already had blocked out

I had my calendar blocked out till end of February 2021 because I didn't want to trade while Booking.com was still getting the administration side of my listing set up.

However, Booking.com over rode this step I'd taken and sent me a customer on Saturday anyway (who I couldn't accept because my property was already full).

Booking.com had already taken the money from the customer and now they want it back asap.

It is Booking.com's responsibility to do this (firstly because they made the mistake) and secondly, because (even if I paid the customer back privately) I will lose money by only being paid by Booking.com my single night rate minus their commission.

So how does this process work?

Who do I speak to? What are their contact details?