Well, this is confusing.
According to the British Foreign Office, there is no Iranian embassy in London, and to apply for visas, we should go through the embassy in Dublin, which a search in the Internet has suggested doesn’t exist either.
So, I had the idea to go through a travel agency that were knowledgeable about this part of the world; I have a friend in the Nottingham suburb of Forest Fields which has a large Islamic population (although mainly Pakistani rather than Iranian), so while I visited her, I popped in to one of them. They say they can help with Iranian visas, but …
The chap called up his visa processing agent and reported back that as a British Citizen, I would have to go down to London and make an appointment with the Iranian Consulate, to have an interview so they can be fully satisfied that I’m not going there for nefarious purpose. He even wrote down the address and phone number (they don’t have an e-mail address).
So, in my pocket I have the address of an Iranian consulate in London that the British Foreign Office denies existing.