Update: We booked an alternative hotel via Expedia and have returned from a great city break in Minsk. We did a walk past the Mercure Old Town Minsk and found it utterly abandoned - all blinds drawn, no lights or power on and no information posted on the front door. The Old Town area is thriving so the hotel stands out like a sore thumb.
We had to chase Accor Customer Services for a third time about the refund and finally received it at the weekend - a month after initially requesting one. We haven't yet received any form of apology or explanation from Accor.