Christ Duncan that doesn't look too good. The other car I spoke about earlier in the post where the whole floor came apart had exactly the same type of body fractures like yours - are you running solid rear subframe mounts?
Unforunately Duncan, your car is a good illustration of what I have been saying for a while - that the stress cracks aren't isolated to the mounting floor pan itself - i.e. it is to do with the load being carried incorrectly through the body structure, cause premature fatigue fractures (i.e. the void in the bottom floor sheet metal).
I'd get that welded up ASAP before it drops the whole floor out of the car (which it will).
Neil333 - one tube is enough to do both rear mounts - If you are doing the fronts, then you need two.
I honestly don't believe the raised lip has anything to do with the fractures - this is not an impact failure where the bushes are moving around so much that they are pulling out of the chassis - but rather a torque load transfer being carried incorrectly through the car causing premature fatigue stress fractures. Like I said, if the lip had anything to do with it, then a quick and easy fix by BMW would have been to simply wack a couple of 3mm washers in between the subframe and the chassis floor - they haven't done that because it is not related to it. Just look at the way Duncan's car is coming apart.
You only drill one hole - in the middle of the floor ridge - the resin is injected into pre-existing holes on the left and right side of that (not the first layer, but the second layer skin - lot of people get this wrong and inject it into the incorrect cavity space).
The link you posted where the guy cut open the sound proofing is incorrect - he was trying to get to the front mounts also - you actually get to those through the front seatbelt mounts - the rear mounts are done through existing holes in the chassis floor (covered by plastic bungs).