I've had a leaky boot for ages. Normally the car is parked facing up the slope, but following Peter Gidden's suggestion, I've had parked it either flat or facing down the slope. Lots of rainy days since doing that, but no leak!
I did try that, yes, but all I ended up with was a lot of dry paper!
I've at least established that it only leaks when pointing uphill, so that's a clue. Quite what the clue is pointing to ...
As is only leaks facing 'uphill' then I would imagine that the water is merely runoff from the engine cover. This suggests that the problem is linked to the boot seal area.
Have you removed this - there could be a flaw in the front part of the lip holding the seal which is allowing water to bleach in.