Right just removing the interior ready for Highwaymans nice leather interior to go in and was just wondering if i'd see (hear) any benefit from sound-proofing under the rear seat?

Not sure why this has been moved to In Car Entertainment?!

Didnt realise external noise (ie Exhaust) classes in that area.

What you looking to sound proof? external noise?

yup

TBH better off sound deadening the doors, rear quarters, boot and the rear parcel shelf. Thar will reduce noise considerably. But if you are removing the carpet a layer of sound proofing felt may help a bit too.

And don't forget the boot lid itself, makes a massive difference imho. Also worth putting and extra two layers or so directly behind the speakers themselves, really makes the difference.