While it's not Hammer Studios, this British horror anthology is jam packed with talent, including Kim Novak [Vertigo], Joan Collins [Dynasty, Tales from the Crypt], and Donald Pleasence [Halloween]. It's a collection of four tales of psychological horror that are hosted by Pleasence who plays an insane asylum doctor who is telling his colleague how four of his patients lost their marbles.
Each of the four stories is pretty even in terms of quality, direction and story. What is missing from this anthology is a sense of humor and decent special effects. These two missing elements rob the stories of their power. There needs to be a little more shock value in the telling of these tales as well as a little more blood splatter!
The best examples of successful anthologies I can think of are George Romero's Creepshow (1982) and Roger Corman's Tales of Terror (1962). Both of these anthologies know how to maintain the right balance between horror and humor. They also contain actors who know what kind of film they are making and don't take themselves too seriously.
Bless the Brits, they just needed to lighten up a bit in Tales That Witness Madness and they would have made a better movie. This one is just O.K. Interesting stories but too tepid of a delivery to captivate their audience.
For more info check out the film's entry in IMDB.