speak to your council Conservation Officer, if they are any good they will advise you on the best solution.
When I renovated a Cat 2 I was able to apply for �2000 a year for external work.
be carefuil with sand blasting, ther are a lot of ignorant thicko's doing it .
Are you sure its granite?
What is the damage ? can it be power sprayed off.
I was told a rub down with vinegar and glycerin is good for a worktop
Have you tires british heritage? what about all the buildings in london that have to be cleaned try asking them.