I used Persian Voyages in the end. Cost £70, but they were excellent, and didn't charge me up front (I would have paid anyway, but it was nice). Got the authorisation number back in 2 weeks, resubmitted my application, and it was all smooth in the end. Very happy.
Did involve a few trips to the embassy though
Thanks to all who responded. For posterity, here is what I did and what happened:
August 13: Contacted www.key2persia.com with form they requested and copy of my passport. Followed up every 3-4 days.
August 30: Authorization number received. Cost US$35, payable when I travel to Iran.
Sept 9: Express mail my passport to Iran interests section of Packistan Embassy in Washington DC (I am in the US right now), arrives on the 10th. (I needed my passport in the intervening week, so couldn't send it in. I also paid the express processing fee of US$52).
Sept 11 (thurs): Called to inquire, told to call again on Monday.
Sept 12 (fri): Passport with visa delivered via Fedex.
As you can imagine, I am very pleased with the way it worked out. Thanks for the advice. Best part was no trip to Washington and passport out of my hands for four days only.
Lawrence