I got given a present two or three years ago called 'The Star Directory' which contained addresses of all well known celebs (pop stars, soap actors/actresses, hollywood actors/actresses), I wrote to about 20 of them and got letters back from all of them. I know they charge for the book (more like about 50 sheets of paper binded!) but it was worth it. I actually still have it now but celebs change their agents quite often so it's probably worthless!
P.S. I would try writing to Sir Alan at his Brentwood office as thats where he's based (my brother works next to his building and sees him often, lucky sod!)