Thanks for the encouraging words folks.
(Picks himself up, takes a deep breath, and starts all over again.)

And yeah; I'd rather find the leaks now than after its been newly upholstered.
And the remedial work can be touched up before the signwriting goes on.
The cliché 'every cloud has a silver lining' springs to mind.
(Although, half a century ago, when I was in the ATC [Air Training Corps,] we were told that clouds have rocks [hills] in them!)

Nigel's using a seam sealer on it, the main cause of the leaks is a seam that runs along the centre of the roof, the full length of the body. (The roof is essentially just two panels with the compound curves at each corner formed and attached separately.)
From this seam the water tracks to various internal ribs, and drips from them.
There are various other relatively minor issues with vertical seams on the side panels, one window, and the 'weather strip' above the windscreen.

We'll make it to the Nationals, finished or not . . . we'll be there!