Update.. The issue of Rate Mixing has been recognised and we have tried a number of solutions to prevent it happening. Partly it may be caused by the lack of fields to set values in our Channel Manager that is linked to Booking.com. A Support agent has manually set the minimum number of nights for us so we can test the theory...
We have also got a phone call due on next week from the Support team who have been looking at preventing Rate Mixing without shutting it down globally
Watch this space
We had minimum stay set and we also set specific check in days, but the booking system ignores them and selects prices from cheaper rates.
For example, a booking for a holiday cottage for four nights should be £107 per night, Booking.com charged them £107 for the first night and only £80 for each subsequent night. For that booking, it loses us £81. Bigger cottages and high season prices will lose even more revenue. Booking.com say they cannot turn off rate mixing and it is up to us to find a way of listing our rates to prevent it happening.
So I have had to temporarily close our cottages until we can find a way round it.