No way to complain about service from Booking.com

I have used Booking.com a few times before and all went well. This year I was arranging a 50th Birthday trip for a part of 8 of us. In advance I booked rooms for all of us at a few hotels, as it was Easter and I wanted to make sure we had options to look at and decide. All of the bookings were cancellable with no charges before a specified date. We choose the hotel and I went online and cancelled the others. I can only assume there was a problem with there system as the cancellation did not register. I did not get any further information emails from Booking.com which you always get as your stay gets closer (do you want car hire? local attractions, directions and reminders). The other rooms had not been cancelled at the hotels , so I ended up being charged as I did not turn up. I have spent hours emailing and on their one and only telephone line speaking to a different front line sales assistant every time. They will not put you through to a manager and they have no process for making a formal complaint against them. Just thought you should know as if something goes wrong, you are on your own.