Elidor is covered a fair bit in the academic piece referred to at the top of this thread. There are radio adaptations of it and Alan Garner's earlier books. (none very good though imo).

There is a very good BBC Play for Today of Red Shift shot at Mow Cop, the iron age hill fort, and a good sixties TV series of The Owl Service which features a prop 'Stone of Gronw' left in place by the film crew

http://ha5.seikyou.ne.jp/home/pemmican/england/owl_service/index_4.html

It is now often mistaken for the pre-historic original which was in fact probably:

http://cy.wikipedia.org/wiki/Delwedd:Llech_Gronw.jpg

Now re-erected after discovery by Jim Perrin:
http://i124.photobucket.com/albums/p32/r_i_c_2007/llechgronwweb.jpg