I made a complaint using the main Accorhotels support page and it was answered quite quickly by the GM of the hotel in question.

The link is: https://www.accorhotels.com/gb/support/contact.shtml

If you go to the 'Contact Us' button and then select 'Your hotel experience' from the dropdown, you can then select 'Complaint relating to your hotel stay'.

I checked into the Mercure Kensington and found that the toilet wouldn't flush and the sink wouldn't drain. The staff didn't move me to another room and just gave me an extra couple of drinks vouchers. Once the GM got involved, I was offered a complimentary night at the hotel on a date of my choice.