SJS is the well known supplier....nearly £2 per rivet. May have to swallow that (though I feel that's still on the 'silly' side.
Brooks have a youtube replacement procedure tutorial.
Head 9.5mm 'shaft' 4.6mm thick, 8.75mm long, 10.75mm overall
The 'Artisan' rivets arn't up to the job I'm afraid.
Thanks for the responses, guys.