There was a knock at the door. I fully expected it to be Sophie, who’d dropped in for a cup of tea and had just gone off to repark her car, but it was a stranger with a letter for our landlord. He had such a nice face that I invited him in, especially as it was pouring with rain and neither of us were sure if he was at the right house, so we decided to establish the facts by the Aga.

He was at the correct address because he’s Bob Bennett, the man called in to assess and then repair ancient monuments all over the world-in this case, the Roman and medieval wall that still encircles Winchester.

I’d opened the door to the founder of the UK’s first Lime Centre, a world expert on repairs to ancient monuments, whose work has taken him across the globe. While he dried off in the kitchen, he explained a bit about his work and why it’s so important. Obviously, it would take a lot longer than the 10 minutes he could spare me, but he had a go and was a natural communi-cator-one of the reasons he’s been a hit with schools and RIBA members to whom he’s been spelling out the basics: cement bad, lime good.

The reason he wanted to drop a letter with us is because we’re currently renting a wonderful 18th-century house in which there are two large cupboard doors. It’s become Alfie’s favourite wager to ask any visitors if they can guess what lies behind these doors-and he hasn’t lost a fiver yet. Open them up, and all you can see is wall, the ancient, original city walls, although they’re badly painted and, in some places, covered in plaster, giving them a look that’s more Hollywood film set than Roman.

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Outside the house, running along the boundary, the wall is on open view, together with the poor repairs carried out in the past-comparable, Mr Bennett said, to a blue cashmere jumper being repaired with red nylon. In order to carry out proper restoration, he needs to take a small handful of the original material, which is then dried and weighed to two demical places. Then, he dissolves the lime with hydrochloric acid, which leaves behind the sand-or whatever aggregate has been used-and, from this, Mr Bennett can work out the proportions he will need to re-create the wall, as it was built by the Romans.

When working with lime mortar, the temperature must be more than 5˚ and rising for 14 days. This may rule out the next few months, but, in the meantime, he may be called in to advise on any number of international landmarks. Just as I didn’t know who’d knocked on the door, he had no idea what was coming when the telephone rang a few years ago: ‘Mr Bennett, we need you here right away, Sir.’ He took the next available flight to New York, where, a limo ride later, he was asked to work on the Statue of Liberty, whose metal framework was being badly eroded by the salty waters of the harbour over which it presides. He assures me the statue is in good shape these days.

He moved on to Windsor Castle following the fire of 1992, but can take on projects of all sizes. Last year, he was called in to help a couple whose cob-walled house in Devon was becoming increasingly damp (because of the terrible paintjob on the outside, which stopped any moisture from getting out). He also worked on a bridge not far from here, which was becoming more and more unstable because the tanks that had trundled along these lanes during the Second World War had dislodged the foundations, causing it to be incrementally on the move ever since.

Mr Bennett is 82. Fifty years ago, he survived a nearly fatal car crash when his life was saved by a passing 10 year old. But that’s another story. He may well have regretted knocking on our door, because it was only with great reluctance that I finally let him leave.

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