As well as most of one field being taken in to the black hole in the chassis, whatever was living in the hole had managed to "rearrange" the underfelt insulation on the engine side of the firewall.
I ordered the required underfelt from Richard at Banks, it arrived the next day, and yesterday, I started the replacement process.
First, you cannot get to the underfelt on the firewall unless you remove the diagonal struts, and the top horizontal cross member to which I believe the emission catch tank used to be fitted.
So after awkward use of a variety of large spanners, the seat belts were removed and the horizontal and diagonal struts came out.
Next the petrol tanks are in the way. Grind off the pop rivets in the wheel arch, undo 4 x 7/16 bolts using a ratchet spanner, and each tank comes through the bottom. An opportunity to de-rust and paint the tanks. De-rusting the right tank reveals heaps of holes in the top which look repairable with fibreglass, as they won't actually take any fluid pressure being the lid on the can.
Next remove the old felt on the firewall. The whole lot is soaking wet, and comes away with a sheet of plastic the full size of the firewall. Behind that there is what looks like fibreboard or thick cardboard. This is soaking wet and falls apart.
It seems that the firewall consists from the seat side as a sandwich of seat - carpet, underfelt, fibreglass, fibreboard, plastic sheet, underfelt, - engine bay.
So after about a hour of removing wet cardboard, I cut out the fibreglass on the seat side which was around the perimeter of the 3 fibreboard panels, and then used a rotary wire brush in a battery drill and a powerful vacuum cleaner to remove all the wet brown cardboard.
Next weekend, make up 3 1/2" plywood panels to fill the large hole, then all the messy fibreglass work can start.
