Got a question for everyone.

How do book a flight online with a "free" stopover?

We're flying to Austria via London. In the past, when I booked the ticket though a travel agent, I was always able to add a few days in London on the way back. This time around, I though I'd try it online since the quotes I'm getting a cheaper, but for the life of me, I can't seem to be able to arrange for the stopover. I can do it as multiple flights (ie: austria london + london home) but it counts it as a separate flight and ends up costing much more.

Any idea?