If you can get hold of it - I think it's kinda obscure - I highly recommend that you dig out this 1973 British horror film forthwith.
It has some of the bad elements of 70s horror but ALL of the good ones, which are massive. I am of the opinion that horror films made in the 70s are the scariest of all.
This has a spooky house, madness, evil plotting, a duaal time-slip storyline (yay), psychotic children and mega glamore. The last element is provided by Joan Collins AND Jane Birkin. In the same film! How, oh how, did I not know that this existed sooner?
Reader, I die.