I did this last year with the €522 flights from DME-DOH-TYO with Qatar. In the end I decided to get a transit visa because I couldn't get good connections with BA from LHR to DME. QR decided to change the DME-DOH and DOH-DME timings delaying the departure by 6 hours which meant I had a 6 hour layover in Moscow on the way out and a 9 hour layover on the way back (both overnight) so I ended up leaving the airport and going to the AirHotel.
For me, £816 is too much for a flight from Moscow if positioning from London, even though BA RFS avios flights are cheap. If you don't get the visa then you could potentially be waiting for a long time in the lounge at DME and it is nothing like Hamad International. And if you misconnect, I imagine it's a bit of a nightmare to sort out and you may end up having to come back to London if you can't get on the following Qatar flight to Doha. If you add in the cost of the Russian transit visa, you might as well wait until the next Qatar sale and pay a little extra to fly from Scandinavia or Western Europe.