Ciaoflyer,
Unfortuntately, the area that you've got the bubbles in will pretty much be double skinned. The inner and outer mudgaurds meet at the outer lip and are fairly close together here. I have taken a couple of pictures of whatyou would see from the inside from a section I cut off my parts car. The first is the rear of the mudguard, you can see two sheets joining at the outer lip, where your bubbles are here is pretty close to the join. The second photo is the leading edge of the rear guard / end of the sill. You can see that again the two sheets meeting at the guard perimeter with a uprisingly large gap thereafter - lots of room for crap to accumulate
