By Jonathan Amos BBC News Online science staff

Even the scale model fills the giant airship hangar

Colin Prescot and Andy Elson will try again this year to take a giant envelope to a height in excess of 40,000 metres (130,000 feet).

They were frustrated in 2002 by the British weather's refusal to give them a suitable launch opportunity.

But negotiations with the Civil Aviation Authority should widen the zone in which their QinetiQ 1 balloon can fly off southwest England and, in so doing, increase the number of days they are likely to get favourable winds.

The project team has been busy this winter checking its equipment. The extra time has allowed for the construction of a second envelope - making slight modifications to the design based on what has been learnt so far.

"This will be the biggest [manned] balloon ever built - ever flown. No-one has done this before so we are learning all the time," Colin Prescot told BBC News Online.

Elson: It is an engineering challenge

A 1/88th scale balloon was tested to destruction this week inside one of the cavernous airship hangars in Cardington in Bedfordshire.

It was pressurised until the seals holding the polyethylene panels of the envelope split.

"The balloon is designed on a computer and it is made on a machine that is computer controlled," Andy Elson said.

"This scale model is our only chance to see what the real thing will look like fully inflated at maximum altitude. We want to see where and how this model bursts and that will go back into our mathematical design model and will show up anything unexpected."

The real thing will be 400 times the size of a typical hot-air balloon and as tall as the Empire State Building; or seven times the height of Nelson's Column in London.

The balloon was tested to destruction

Prescot and Elson will be making their ascent on an open flight deck - Russian spacesuits will have to protect them from the elements. The whole trip should be completed within nine hours; they carry oxygen on board the platform for 11 hours.

The duo will remain seated all through the flight but they will have only limited time to enjoy the view. There are radiation and micrometeoroid experiments to be performed and Prescot will try to fly a solar-powered propeller-driven plane in the stratosphere.

"It is going underneath the gondola, carried up on a line," he said. "It's got a 36-ft wingspan but weighs less than 10 kilos and has a TV camera on the end. So we've got a lot to do and keep us busy."

The team is confident that given the right weather, the QinetiQ 1 balloon will smash the current altitude record of 34,747 m (114,000 ft).

Prescot: It will be inspirational

"What we've done through the winter is make refinements that will hopefully take as many problems as possible out of the loop so that on the day we have more latitude if things go wrong," said Elson.

June to September is the proposed launch window. There are several parachutists who are also planning to take balloons into the stratosphere - but the official Federation Aeronautique Internationale rules demand pilots bring down their craft (the parachutists, if they go, clearly will not be doing this).

This leaves Elson and Prescot a clear run at the record.

"This is a great adventure - the ultimate challenge for a balloonist," said Prescot.

"The idea of sending back images of two little spacemen on an open deck underneath the biggest balloon in history with a big black sky and the curvature of the Earth underneath us - that will capture the imagination."