Aerofoil,
Download the form SRG 1160 from the CAA website as GoldenMonkey mentions. This form needs to be faxed TO the CAA , they process the form for a fee and then send direct to the FAA a pre-release of your details. There is only one person dealing with this form at the CAA so be patient, he works as hard and fast as he can. Mine took less than 2 days.
When you fax the paperwork to the FAA CALL them to confirm that they have recieved it, better to be safe than sorry.
They (the FAA) require the name of the Local FSDO office where you intend to collect your endorsement, because this is where your information will be released to. So, therefore if you change your mind and go to a different FSDO office, they not only wont know who you are but wont have any of the information. You will have to re-submit your request to the FAA. If unsure of the local FSDO call the flight training school you intend to use.
Also don't forget to call the Local FSDO office before arriving for your endorsement, you need to make an appointment. They don't appreciate people just turning up.
But it isnt as bad as it sounds and the whole process only takes around 4 to 5 working days, so fly safe and enjoy
WW