Sapphire & Steel
I was really young when this was aired but I bought the series on DVD and despite the budget and special effects being non existant, even watching it now it's really dark & claustrophobic and creepy as hell, and the scripts & ideas behind it were fantastic.
It's about 2 agents (Sapphire & Steel) who are a part of a group of mysterious agents who fix cracks in time.
It's never explained what or who they are or who they work for, they just turn up & stop time manifesting itself as either ghosts or creatures or making deals with people giving them chances to turn back the clock and correct mistakes that give time a way in to threaten the world.
My favourite story is the one where time manifests itself as a faceless man who can transfer himself into any photo, trap people inside photos & make copies of people inside them.
Have a trailer of that story...