Hi all,
Many thanks for the replies. I've tried to sort things out over the past few days:
1. I called the switchboard 020 number for the embassy - just got a long recorded information message and a pointer to the expensive operator assistance number. Not much help there.
2. I called the US Dept of State. Eventually spoke to a rather sleepy lady, who put me on hold for another 5 min but eventually reported back that the date they have on file is for the 12 March - my 'second date'.
3. I tried to send a brief, concise email to LondonConsular@state.gov. I clearly marked in the subject that I required assistance. However, I was immediately returned a default message saying that my email was deleted:
"This e-mail address is solely for individuals who have been authorized
to contact the Consular Information Unit about a specific matter. This
address is not for general inquiries from the public, attorneys or
agents of record. Please understand our workload does not allow us to
read unauthorized inquiries, but the Embassy has set up several
mechanisms to provide general and personalized guidance"... and so forth

again referring me to the £1.20 number!
So I'm going to ring the premium number tomorrow. Unfortunately I work away from home during the week and have no access to a landline - don't think you can ring premium 090 numbers with a mobile phone so I've just asked my Dad to ring for me! Hopefully the fact that it's him calling and not me won't be a problem.
To babblesgirl - I'm so sorry I cant be more help. I havent been checking the boards frequently since our NOA2 so I didnt know you were having such problems getting an interview date. I think the switchboard premium number ppl will tell me to come on Wednesday 12 next week. But even if this was free i'm not sure how I could help you at such short notice. Both appointment letters have my name, along with the date and time, and I dont think the security ppl would let you in with my letter, if i mailed it to you. What does anyone else think? I've been through the visa process at the embassy twice before a couple of years ago (for a J-1 and choir tour visa respectively) and they were super strict on the gates. But I'm willing to mail it ASAP if this will help.
Thank you all for your guidance - invaluable as always