Mine went to the point of it looking like a piece of lacework at the rear offside so I had a new chassis put on it last March. Yes, it cost a bit but every penny was well worth it. I am delighted. To be honest, I do not think it was expensive for what I got. I would recomment doing it to anyone who wants to get rid of a 'lacework D2 chassis!
I had been getting advisories for about 4 years showing 'slight corrosion to the rear chassis'. Their idea of 'slight' and mine were two very different things!!
The pictures of mine being done is in a thread on here, somewhere!
P.S. I am not of the age to get under and do the jobs myself so I had to rely on other's advice.
i would of cleaned/cut a bigger area up if it was me! then fired loads of waxoyl and old sump oil down the chassis both ways as you would of had a bigger hole to work with, then welded the patch in