Any question answered...

Right, so Booking.com issues.

I tried to book a place, and several time I get the "invalid card" message despite it being 100% correct, no issues with bank, tried partner card and got the same message.

Is this something that hotel/b&b cheekily marked as invalid to force us to pay on the spot instead of waiting until the day of the booking?

Thank you for the booking you have made through Booking.com.

Due to an error, your booking was not fully confirmed on our system by Booking.com. We have therefore created the booking for you as it was booked at booking.com.
Due to this error, we were not able to charge your credit card, this is why we kindly ask you to call us within 24 hours to proceed with the payment.

PLEASE NOTE: even if you update the credit card, payment will not be taken, to keep the booking, payment needs to be done by phone.