Try The Innocents.

Here are some other fantastic creepy atmospheric horror gems...

Diabolique (1955)
Repulsion (1965)
The House That Screamed (1969)
Let's Scare Jessica to Death (1971)
A Warning to the Curious (1972 BBC TV Film)
The Stone Tape (1972 BBC TV Film)
The Legend of Hell House (1973)
Amityville II: The Possession (1982)
The Woman in Black (1989)
Ghostwatch (1992 BBC TV Film)
Dark Water (2002 Original)
The Uninvited Guest (2004)
Sauna (2008)

Wow, god damn - just watched The Stone Tape. It's a perfect example of what I want. It's shortcomings are clear, in that it's a made for tv film, but the aesthetic and performances more than make up for it. It made me feel sick to my stomach and gave me a legitimate scare, which is rare for me, and much appreciated. The plot synopsis intrigued me, but I wasn't expecting it to have the effect it did. The sound-work in particular stands out to me, and I've not seen credits (of all things) so perfectly underscore dread before. Thanks very much, Horror4ever, going to try some of your other suggestions soon.

Glad you liked Stone Tape. Try those other BBC made films...they all are great, hope you'll like them too.