Roosie - while you're on welding it up, take the rear bench out and look down at the sides of the floorpan - you'll see some oblong plastic covers...
Pull them out and have a good look inside - you'll be looking into the insides of the chassis rails. They ALL rust here sooner or later, mine looked mint from outside but were starting to rot slightly from the inside out
The Youth welded them up easy enough for me, but you really need the rear beam out to get at them to weld.
If all's good in there then get a good squirt of Dinitrol Cavity Wax in there
Neil.
PS on the plates you are going to weld for your MOT, make sure you don't block the drain slots up (you can see them in the bottom right of your pic) - This will have been where your rot started IMO...