Britain's oldest surviving medieval external doors are set to be unveiled following months of painstaking restoration and help boost a campaign to revamp a historic King's Lynn chapel.

Work to restore St Nicholas Chapel's 600-year-old doors started in March as part of a bid to restore the building to its former glory and last night it emerged the refurbishment will be finished by Christmas.

The announcement by the Churches Conservation Trust (CCT), which owns the chapel, came after a new exhibition was opened at a King's Lynn museum yesterday which looks at the history of the building and its architecture as well as the saint himself.

'The doors are a masterpiece of late medieval carpentry, probably completed just before 1400 but now showing the effects of 610 year's exposure to salt westerlies off the North Sea,' said Brian Clark, CCT's conservation project manager.

'Our studies showed that the first colour scheme comprised vibrant green and red, to offset their decorative carving against the structural background. After long consultation and review, we have decided to revive the original finish in specially-mixed oil paints.

'Lost elements at the base have been re-carved where these were vital to safeguard the historic timber, but otherwise we have simply conserved original fabric.

'The final colours are due to go on between now and Christmas, and will then harden under cover for another month. The doors will be unveiled for all to see at some point in the new year.'

Adrian Parker, secretary of St Nicholas Friends' group, added: 'Visitors to the chapel during the autumn have been able to see the undercoat colours – which pick out the tracery on the doors.

'But now we are ready for the red and green final paint – it will be a revelation that a medieval door looked like this. It is very exciting'

Conservationists stripped back 17 layers of paint before every hand-made iron nail was removed, cleaned, treated and carefully replaced.

The doors will be painted in the original medieval colours – red and green – but might not be unveiled to the public until next month, a CCT spokesman added.

Yesterday's opening of the St Nicholas Chapel exhibition at True's Yard Museum was timed to coincide with St Nicholas' Day.

Officially opening the exhibition, West Norfolk mayor Geoffrey Wareham said: 'Every time I go into that chapel, I'm in awe of its splendour.

'It is part of Lynn and it's part of the town's heritage - we can't neglect it and we can't let it slip away.

'I can't thank enough the people who have put in so much effort to restore this chapel. It's been a tremendous effort so far.'

At the centre of the exhibition is a model of the chapel which retired carpenter Fred Hall, 79, spent more than 500 hours over five months building.

The West Winch resident spent hours taking countless pictures of every aspect of the building as well as carrying out his own measurements to help him create the model .

He said: 'Everyone has been saying that I must have a lot of patience but it's just something I love to do.'

Historian Dr Paul Richards had set Mr Hall the challenge of making the model after being impressed with his models of the Lynn Minster and the town's Custom House.

He said: 'I blew up the pictures of the chapel in the 18th century so Fred could include the original spire in his model rather than the one we currently have, which was put in place in 1869.'

Dr Richards continued: 'I think it is a balanced exhibition with the information panels surrounding an impressive centre piece.

'It will provide people with a brilliant snap shot of the chapel and I hope it will raise awareness of the chapel's appeal.'

He added: 'There's been a lot of pressure getting this exhibition ready in time for St Nicholas Day.'

Lindsey Bavin, museum manager, was tasked with pulling together the information for the exhibition's panels.

'It was hard work but it was a labour of love because I love this kind of thing,' she said.

'The chapel has got a fascinating history and I hope this exhibition will raise the awareness of St Nicholas Chapel.

'It is on the edge of town so I think people tend to forget it's there but I hope people come along, learn all about it and help to save it.'

The opening of the exhibition comes after TV presenter Loyd Grossman appealed for more people and businesses to come forward to help the fundraising campaign.

The Through the Keyhole star is chairman of the Churches Conservation Trust (CCT) which is working with the Friends of St Nicholas to raise �210,000 which is needed to secure Lottery funding.

The appeal is currently just �26,000 short of its target and fundraisers have got until early next year to raise the remaining funds before an application for the �1.5m grant is submitted to the Heritage Lottery Fund.

The American-British TV star has described the chapel in the heart of Lynn as an internationally important jewel in the crown of the trust.

The �1.5m project will include �800,000 to re-roof the nave and south aisle while adding insulation. A further �130,000 is needed to provide toilets and a kitchenette area, while �120,000 will provide a heating system and �30,000 will light the beautiful carved angels in the ceiling.

If the application to the Heritage Lottery Fund is successful, work could start on the chapel by next September before reopening the following May.

One of the largest events held to raise money for the cause was the Festival of Angels which saw individuals, companies and organisations create some 81 angels which adorned the historic building for two days in June.

There were angels made entirely of recycled materials, one made out of vegetables and another made out of old copies of the EDP which all made for out-of-this-world viewing at the special festival and helped to raise �7,000.

The plea by Mr Grossman comes after the historic chapel was given the green light by borough councillors to have 92 solar cells on its roof as part of the major refurbishment scheme.

It is believed to be the first time solar panels visible from the ground have been allowed on the roof of a Grade I listed place of worship.

The installation of solar panels will allow for the provision of heat and lighting to the church and will have a huge impact on future running costs of the medieval building, the CCT has said.

The trust has 341 buildings and St Nicholas' Chapel is its biggest. The trust has managed the building, which is still consecrated, for more than 20 years.

Any donations to the appeal can be made at True's Yard Museum, close to the chapel.

For more information about the appeal to restore the Lynn chapel and the Churches Conservation Trust visit www.visitchurches.org.uk.