They don't need to be flown vigorously and frequently, just an airing of three or four flights a year. “The value is definitely in them being airworthy. It is the best example there is from a history standpoint, as well as being a lovely 10-out-of-10 restoration, probably as original as it is possible to get,” says Grace.

“MH415 is an expensive airplane at £4.5m. In your gut you know something's right, but these days it's evidence, evidence, evidence.” That'd add half a million quid or so, but that’s one of the perpetual frustrations of dealing with anything historic. “All it'll take is some random piece of paper coming out of somewhere with the serial numbers of the engines fitted to all the Spitfires in September 1943. “I've spoken to every historian and gone through every record over several years, but I cannot find any evidence to confirm that one detail 100 per cent. God knows what a matching numbers aircraft is worth,” says Grace. “There's probably a difference in value of £1m between an original Spitfire and non-original. As for MH415’s own niggle? Its restorer Richard Grace is “99.9 per cent sure” that the engine currently powering this Spitfire is the 1943 original.

Many high-end collectible cars or motorcycles, for example, carry with them myths and legends about their creators, their owners or their adventures – stories that add value both at the dinner table and in the sale room.

But part of anything and everything collectible is that little niggle of mystery or uncertainty that journeys with it.

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