"It leaks only when driven through a heavy rain: it will not leak if left in a down pour for hours."
If this is what you find conclusive, then you may have to check 2 places.
Hoist up your MB and look for holes undercarriage, esp at the area where it leaks. Don't discount the firewall at the proportionate area where the leak is found.
If it was an uneventful search, then you might have to check the skirting where the door closes onto the step. There could be a gap where water may be seeping in - more so when travelling but not stationary. This is a wild guess.
I would go for the hoisting up, thus you would get a very definite clear picture of whats lurking beneath.
All the best.
126 tailed by a 203, 129 leading the pack.