Dan Spokes knew exactly what kind of vehicle he wanted to drive on his long-anticipated overland adventure to Asia. It would need to be sturdy, reliable and spacious. Four years ago, in a garage in Bristol, he spotted exactly what he had been looking for: a double-decker bus. After handing over £2,500, he emerged the proud owner of a 1981 Bristol VR.

Though a Land Rover might have been a more obvious choice for such an arduous journey, Dan, who runs a business restoring classic cars, relished the challenge of transforming this suburban bus into a lean, mean expedition vehicle.

'It's unbelievably reliable and easy to drive. We had thought about buying a London Routemaster but we test drove one and it was hopeless, with a top speed of just 35mph,' says Dan. 'The VR can do up to 70 miles an hour.'

He recruited friend Matt Lumber and they rebuilt the vehicle. They even turned the bus into a 'convertible', creating a fold-down top deck which can be removed to cope with low bridges and which will also act as the bus's sun terrace when stationary.

The interior is perhaps even more radical. The seats have been stripped out and replaced downstairs with a kitchen, shower and flushable toilet. Upstairs, there are three double cabins. Reminders of the bus's previous life have been left in situ, such as the sign warning 'Passengers must not speak to the driver without good cause ...'

Previously operated on the Bristol-Bath-Weston-super-Mare route, the revamped VR's next journey will be somewhat more adventurous. Next Sunday, Dan and Matt will leave Bristol for Dover and a ferry. They will turn left at Calais and head east through Europe, the Balkans, Turkey and Asia until they hit Nepal, returning in a loop via Central Asia and Russia. The trip will cover 22,500 miles and pass through 25 countries. Dan and Matt will be keeping a video diary of their adventure which you can see on their sponsor Expedia's website (http://letyourselfgo.expedia.co.uk/doubledecker/).

Dan admits the logistics of planning the trip have been a 'nightmare'. Aside from the basics such as obtaining an MOT and insurance for the bus, Dan and Matt have had to get visas for the vehicle and themselves for several countries as well as studying weather reports, maps, road conditions and the changing political situation in countries such as Pakistan and Iran.

They are planning to meet friends at various points along the route - including a rendezvous with Dan's fiancée Fiona at the Taj Mahal - and they expect to pick up travellers who they meet along the way. 'It's not a commercial venture. If people want to come on board the bus, we'll ask them to make a donation towards the fuel costs and we'll also be accepting donations for our nominated charities, the David Shepherd Wildlife Foundation and Mountain Rescue,' says Dan.

As the departure date looms, Matt and Dan are finally allowing themselves to get excited. 'The bit I'm looking forward to most is driving the bus along the Karakoram Highway through Pakistan,' says Dan. 'I've got this image of us with the top down drinking sundowners in the Himalayas.'