G-BOAA there is no need to go through immigration, customs, check-in etc. Just go to the transfer desk between security and arrivals when you arrive and let them check you in for your next flight and retrieve your luggage and re-label it to your final destination. Then go through security into the departure area. I am told it takes about 15 minutes.
This is also covered by the 'About DOH airport' widget on the dashboard.