You should ideally lay the Mini on its side. Cut the floor panel (with a
nibbler) just about an inch from the battery box. Make an edge with a
Joddler on the new panel.
Perhaps the floor panel is rusted under the re-enforcing sheets (thicker
than normal panel; I would say 1.5 mm). You can make the re-enforcing sheets
yourself from 1.5 mm sheet steel. They are there to give strength to to rear
subframe mounts. Think ahead befor you act. When you weld something in, you
can not, or at least very difficult, make other edges ready for further
welding. Leave the boot and the boot rubber in place, so everything will
look good after the job is finished. I use zinc spray after welding. Clamp
the panels tight before welding. This is very important. I use self tapping
screws and after welding and removing the screws I fill the hole where the
screw was with weld creating a spot weld again. Use 2 planks of wood which
you clamp together and place them between the boot floor place which is to
be welded and the underside of the parcel shelf. I suppose you removed the
rear subframe already? Clean it and paint it. I used 2 layers of Hammerite
black. Look at the tops of the rear shock breaker mountings. Are they
rotten? More work...

I hope you can do all this in a weekend... Success!

If you are using the full width panel for the boot floor (the one that is
about 12" wide and just dips into the spare wheel well), you will find it
has to be fitted with the valance removed. Not a problem since you are doing
this too. The bumper lip is made up of 3 thicknesses. The rear panel lips
out on top, the boot floor extends under this and the rear valance lips out
underneath. It is this lousy design that cause the rust!!

I would recommend cutting around the top of the spare wheel well and lipping
the repair section into it. This is much easier than trying to follow the
floor down into the well and back out again.

Have you bought the corner closers for the rear valance too? If not you will
need to make up sections for these.

Drill out the spot welds in the bumper lip from above. If you are Mig
welding, you can use the holes to puddle weld through to the floor when you
reassemble.

hi
dont do as i did and paint the floor before you fit the fuel tank bracket,
also its easier to coat the new floor in underseal before you weld on the
valance but its highly flamable.
The joint between the valance and the floor really need a 99% pure zinc
spray to stop corrosion because paint will burn off with the welding.

fragged