Midnight Court

Useful Joan Aiken related links - if you have any others, please add in the comments & the links will be added when the post is updated.

General
The Wonderful World of Joan Aiken
The official website, with lots of info etc. (Includes a useful bibliography that can be sorted in different ways, including chronological and by series/age ranges

Joan Aiken entry on Wikipedia
Joan Aiken obituary from The Guardian (Jan 2004)
Joan Aiken obituary from The Independent (Jan 2004)
Joan Aiken obituary from The Telegraph (Jan 2004)

Amazon has a Joan Aiken page.

Illustrators
Sadly, I can't find very much for the late Pat Marriott, other than Joan's own tribute (on the official website): "She has an absolutely magical gift for presenting landscapes and characters just as they were in my mind. She can get wonderful sympathy and terrific action and pace into her drawings of people; with some of my characters now, such as Dido and Miss Slighcarp, I can hardly envisage them other than as Pat draws them."
Jan Pienkowski
Quentin Blake

The Wolves of Willoughby Chase Sequence / Wolves Chronicles
The Wolves of Willoughby Chase on Wikipedia
Black Hearts in Battersea on Wikipedia (includes info on the 1995 TV adaptation)

Arabel & Mortimer
Arabel's Raven Fan website.

TV/Film Adaptations
The Wolves of Willoughby Chase on Wikipedia and on IMDB (1989 Suart Orme film adaptation starring Stephanie Beacham, Mel Smith, Emily Hudson and Aleks Darowska).
Black Hearts in Battersea (1995 BBC TV production by James Andrew Hall, starring William Mannering and Jade Williams as Simon and Dido). (Some info on the book entry, above.)
Midnight is a Place on IMDB (1978 ITV production by Chris McMaster starring Simon Gipps-Kent, Maxine Gordan, David Collings, William Squire, Lally Bowers and Ron Moody.)
Arabel's Raven (Jackanory, 1971; read by Roy Kinnear)
Arabel and Mortimer Wikipedia entry (1993-1994 CBBC puppet series)

Fanfic
Joan Aiken Wolves fanfic on AO3 (A very rare fandom, but there are currently 3 stories, mostly via yuletide).

Icons, graphics & fanart
(No links as yet.)