Here's the inside of that inner wing bodge. It's lucky the car didn't go up in flames - the carpet is badly scorched because it wasn't removed before welding.
A close-up of the corner where the sill, wheel arch, and inner wing meet up:
I should know better than to lift carpets up!
A rotten-through patch in the floor over the nearside crossmember:
A much worse one on the offside:
Whoops, I seem to have taken a screwdriver to the rust:
In other news, I discovered the reason the heater wasn't working very well was the power connections hadn't been swapped over when it was converted to negative earth, so the fan was going backwards. I've made a start on replacing the leaky water pump and flushing the rust out of the coolant system. For some reason it was fitted with a 68C thermostat, which also can't have been helping the heater output.