Sending Mark Fanmail? 

The Address is

FAO: Mark Gatiss
c/o Curtis Brown Group Ltd.
Haymarket House
5th Floor, 28-29 Haymarket
London SW1Y 4SP, UK

Including a SAE, or international postage coupons if you’re not in the UK, will increase your chance of a reply (that was he doesn’t have to go to the post office!)

Good advice from Ian re- Fanmail

The main thing to remember when sending Mark fan mail is have patience. 

This man is extraordinarily busy. Our rough estimate is a 4-8 month wait. This is based on others who have received mail. Don’t let it put you off, just expect to have to wait a while.