The aluminium body section where my near side tubular outrigger is attached has turned into powder in a small section (see picture). Can someone please provide some technical advice on how I should fix this. Is it a case of angle grinding it out and covering with a replacement section? If so, what material should I use and what thickness? How much of an overlap should there be? Thanks.

i used the YRM repair panels for this.

made the job a breeze ill be honest (once i stripped out enough to get at it )

That aluminium section is 2-3mm thick and plays a major part in holding the tub in place. That's why it is a bolting point to the chassis via the outrigger.
I've got the YRM repair panels, as suggested by Top Drive, ready to go in when mine gets put back together.