The rusty metal below the surface is the original boot floor. The new hole is in a previous dodgy patch - probably water leaking into the boot has got between the two skins and rusted the patch from the inside out. I didn't want to take her off the road to do a proper repair though so I've reluctantly bodged another patch on top of the old one:
[insert excuses for the horrible ugly weld here: poor access and visibility, working upside down with molten metal spattering onto my arm, out of practice, welder playing up due to lack of use and storage in a damp barn, welding onto old rusty thin metal, patch made from a bit of old rusty door hinge, etc.]
Hopefully it will be good enough to get her through the test. Once Fenchurch is back on the road I'll have the opportunity to lavish Lin with a bit more attention than she's been getting recently.