I used roundtheworldflights a couple of years ago for a trip with a slightly unusual itinerary. I tried in vain to arrange it first with Trailfinders and another well known company. While there are people there who who might know how to do this, I struggled to find them! The quote I was getting went up to around £4000. It wasn't until someone advised me to check out rtwf that I managed to arrange the tour, settling on a skeletal long haul ticket (no one else had mentioned it as an option) and buying tickets as I went along to afford me maximum flexibility. I also saved over a grand. Trailfinders for me had zero value-add and slapped on £50 for a ticket on a Bolivian airline because the site was in Spanish. I've been recommending rtwf for their professionalism ever since so I am happy to do so publicly. And Simon was a pleasure to deal with!