This is going to be unfortunately vague (I read these books as a kid...some 15/20 years ago).
What I can remember is that the stories/books had a dark atmosphere, that the main characters were young, that they found some kind of wormhole in a garden that would take them to another place.
I've searched online, tried looking at lists of books published around the time...nothing!
Can anyone help?
Additional Information: I'm sure there were a group of them - all children/teenagers. Mix of genders. Setting might well have been England, but might have been elsewhere. The only other detail I remember is that there was a series of books about this.