So I know rear arches rust on the l322. Mine has some bubbling of paint on the nearside. None on the offside. Both are rusty behind/underneath.

Whats the likely extent of this...? (I know you can't really tell until you start poking... but i resisted a poke)

Definitely take the inner wheel arch covers off , I opted for a £3k restoration incl sills on the H&H at a top body shop/restorer to keep the H&H in tip top condition, it wasn’t until the sill covers and inner wheel liners came off the true extent of the rot was revealed Holland and Holland Range Rover By Overfinch

Get a body shop to put new metal in there. Mine cost £1000 for both rear sides.

same as john 61 you will have to remove rear arch covers and sill covers to see what you are dealing with they rust bad on the front of the rear arch behind the cover and wear jacking points are when i done mine i replaced sill part with 5 mm plate done a viddie job new vehicle purchase
2007 supercharged 4.2 java black .sand leather vogue se trim
previous cars .
range rover classic 200 tdi
discovery 1 200 tdi
freelander 2 2008 hse
discovery 1 300 tdi
p38 4.0 ltr man r380 ex royal support vehicle on v5c
discovery td5 man off roader

Are you sure that is not just heavy surface rust? Could you use, for example, a dremel with a coarse grind stone to see if you can get back to shiny metal?
Mine looked similar but there wasn't any actual rot.

Mine were similar had both side done rust cut out and new metal repaint including rear and front bumper for 900 quid