I am UK based and booked accommodation in Boston through Booking.com. The accommodation provider cancelled shortly before our stay was due to start and promised a full refund.

After waiting the 30 days prescribed by Booking.com, no refund has been forthcoming. I have chased this by telephone and email with the provider, and with calls to the Booking.com helpdesk, but no refund has been made.

What is the most effective strategy for pursuing a refund from an accommodation provider who cancelled our stay through Booking.com?