"If we can't find the right buyer, we may well carry on regardless as a private Millennium project of our own."Magdalene House is being sold through Southams, 8 Market Place, Oundle, Peterborough, Cambridgeshire (01832 273565); asking price pounds 245,000. It is a place of such fantasy, set within an 18th- century landscape (which, thanks to Fuller's estate, remains anachronistically intact) that one hopes that someone will be prepared to lavish on it the attention it deserves."I have very mixed emotions about selling it," he says. During his investigations, Mr Longmire discovered the mechanism for a large telescope beneath the floor of the sitting room, and he has gained listed building consent to reinstate a camera obscura in the dome.Mr Longmire describes the Observatory as "a listed Grade 11* star-gazer's dream in need of care". There is also a conservatory on the south side of the building. He studied plans for a similar but not identical building in the collection of the Royal Institute of British Architects and gained access to records of Fuller's accounts, which showed payments to Turner for his painting of the Observatory (part of his Sussex series of watercolours), but not to Smirke.The Observatory had been lived in by several previous owners and around its semi-circular courtyard are rooms that, according to the Longmires' plans, would constitute three bedrooms, with the kitchen restored to its original site where the large fireplace and salt store still exist.

He is clearly both passionate and knowledgeable about the period and, while the Observatory fulfilled his criteria of "a small house with large rooms", it was piecing together the historical jigsaw that gave the acquisition its edge There are still enigmas to be solved. Fuller was a patron of the arts and of scientific research, and the Observatory was clearly built to be used. It is based on the same principle as that of the Old Royal Observatory in Greenwich: what is now the sitting-room has a section of roof and walls designed to be opened out to gaze at the heavens; the dome unfolds to facilitate the use of a camera obscura.The history of the Observatory as related by Paul Longmire reads like a cast list of luminaries from the early 18th century. As with the Gents, restoring an historic building had been a long-held ambition, but, having some health problems, they now feel that have begun too late and hope to sell it before work starts. Listed building consent is now in place for most things that need to be done to restore the Grade II* listed observatory, one of the most extraordinary buildings still to be found in private ownership.Crowning a hill with 360 degree views of the Weald of Kent, the South Downs and the sea, the domed building was designed by Sir Robert Smirke, architect of the British Museum, and built in around 1812 for the flamboyant landowner "Mad" Jack Fuller. In doing so they admit that they feel a responsibility for handing over such a unique building to someone who will care for it as they have done.It is a sentiment that is shared by Paul and Eleanor Longmire who bought the Observatory near Brightling, East Sussex, two years ago. "Everyone who worked on the place loved the challenge, and we had no problems in finding the people," Mr Gent says.

The specialised work included plastering using the original techniques, re-roofing with local Collyweston tiles and salvaging Comper glass from damaged windows and re-using it. Kerrou was so delighted that he recommended the building for a Civic Trust Award.The Gents have now decided to sell up and look for a new challenge. An expert from the Council for the Care of Churches helped them with historical detail and the council's conservation officer, Michel Kerrou, directed them to skilled craftsmen in the area. They were invited to an exhibition at the Heinz Gallery in 1986, where the architect's designs for a chalice and paten for the chapel were exhibited. Water and electricity were installed, a whole new row of windows was set into the ground floor in the style of the existing high windows, and pews were made into window seats and a bookcase.Working from home, Mr Gent enjoys the full benefit of the painted ceiling with its Latin inscription: "Everyone who walks into the room says, 'Wow!' I love working in such wonderful surroundings and it is also an advantage to be able to invite clients to meetings here."Once they had embarked on the project the Gents found people approached them with advice and information.

To their great relief it proved a risk worth taking.The project was an exciting one as it involved making imaginative use of the existing features. The planning authorities conceded that certain major alterations would have to be done to make living in the chapel feasible. They did, however, have to commission an architect to prepare full plans in advance, which ran well into four figures. Northamptonshire County Council, to whom it fell when the workhouse closed in the Fifties, had not had the resources for its upkeep and finally offered it for sale. The Gents were competing against a builder who wanted to convert it into two dwellings, so their proposal to keep it intact as a single home met with approval. When we walked into Magdalene House we felt an overwhelming feeling of peace, and that sealed it for us".The building was commissioned as a workhouse chapel by the Guardians of the Poor of Oundle and completed in 1896.