Julia Brookes of Times Travel's Readers' Forum responds: If you’re looking to get there quickly, the only real option is to fly. Vietnam Airlines has a direct route with tickets costing about $190 (£100), or to save money you could travel overland to Bangkok, which would take a day and cost $20 and use a low cost carrier such as Tiger Airways or Air Asia which would charge about $60. All of these can be booked locally.

Alternatively, Nick Pulley, from the specialist Indochina tour operator Selective Asia (www.selectiveasia.com), recommends the scenic route. This would mean a ferry down the Tonlé Sap Lake/River to Phnom Penh where you would probably want to spend a few days. From here you could take a boat as far as Ho Chi Minh City (formerly Saigon), departing Cambodia at Kaam Samnor.

Once in Ho Chi Minh City you can reach Hanoi by the Reunification Express train (journey time about 30 hours) or do the sensible thing and take plenty of breaks at places such as Phan Thiet, Hoi An and Hué. You will have to get another visa because the one you have is a fixed single entry. Buy a visa in Cambodia for $30; it should take only about 15 minutes to be processed.

How to get from Siem Reap to Hanoi really depends on how much time and money are at your disposal.

Julia seems to have covered most things. By land you can travel from Siem Reap through Laos, or take the much longer route of Thailand.
In order to go the Thai route you would need to get from Siem Reap to Bangkok which would take one day, then head North to one of the few Thai/Laos borders. From there head to the capital, Vientiane , find a connection to Hanoi, which took me over 30h by bus. If you hired a local tour op to take you by a more direct route (I.e cutting out Bangkok) it could be quicker but you would still be covering at least 700 miles to get back to Hanoi and crossing Laos.

Though I found this a great experience it is time consuming and the costs of visas and other travel expenses do add up.

I know little about how to get from Siem Reap to Hanoi via Loas and ignoring Thailand, though it looks quicker on a map the logistics my be more complicated. However it maybe worth you looking in to.

I would suggest you use the rail and bus to get from Hanoi to Siem Reap stopping at some of the places in Vietnam already suggested Hoi An, Hue, Dalat or Na Trang then fly back from Siem Reap. By trying to get back to Hanoi via land by a route different to the one which got you there you are doubling your trip (Obviously!), which is great so long as that is what your prepared to do.

Enjoy, J