Tried to get the backer rod into the boot seal. It's too damn tight to get all the way round and the backer rod too delicate. Moreover, the bottom corner each side has an extra tube of rubber inside the seal so it is doubled up but more importantly has a restriction. Broke one of my rods that I use for threading electrical cable. An utter nightmare of an afternoon but it is possible to get shorter lengths in at the corners. 15 mm fits inside the boot seal if you enlarge one of the existing holes and with one of said rods and sone Vaseline feed the backer rod through. Done one side and will see how it performs. Looks like we've got plenty of rain on the way to test it!