Yes, Hoppy's link is very interesting and everyone loves a mystery...
But when you start to think about it, the story doesn't hold up. Why would the British Government 'cover up' a foiled invasion attempt ? Why would trainloads of burned survivors be taken all the way to Berlin ? And so on....
But Sapper's right - beaches and sea areas were 'fired' ; there is a photo of such an exercise on the dustwrapper of Peter Fleming's
'Invasion 1940' for instance.