If the official QR MCT has changed to 45mins, that is new. I had a 35 min connection on a couple of LHR-DOH-BKK flights last year, and made all but one (due to a late arrival).

If you know DOH well and are in J or have access to Priority Security, 45mins is fine. As long as you arrive on-time, you'll be fine. Just follow the signs to Flight Connections, and follow the procedure.

If you are a newbie to DOH, and are in Y, then 45mins might be a struggle.

It is worth noting, that if you have a tight connection, QR will send a representative to the jet-bridge when you arrive, and they will escort you through.