Thanks all!

I called the BAH line a couple of times, got disconnected both times. So I cancelled on line as mmx suggested. Booking is still showing in MMB, but took screenshot so hopefully that is ok. I have only paid deposit so far, but don't want to be liable for the balance, so hope that cancellation has gone through. This the first time using BAH, generally I prefer the flexibility of flight only and hotel booked separately, but double tier points too good to miss!

Now need to rebook ...