This explains how to remove the bumper.
If you look at the two photo's in the above post, look at the bottom RHS corner you will see a piece of sticky tape over a square hole, put there during manufacture .
That is my source of the leak !, On the LHS vent the tape was coming off and the RHS vent the tape had gone and a open square hole was there, both letting water into the boot.
I cleaned well around the vents and put plenty of silicone mastic around them and sealed the two holes up.
To make it easier to remove the fasteners in the wheel arches I removed the wheels.
I gave myself a day to do it.
Good design Ford, you leave holes for the water to get in .
B-Max, 1.6tdci, Titanium, Blazer Blue, 2013