Derelict Sunday - What is it? (I don't know!)

Not far from our abode there used to be an abandoned house in an isolated corner between two railway tracks. Some years ago it was demolished and the site remains derelict. This weird object is on the site of the old building itself.

I'm wondering what it is - ?maybe the input to an old septic tank perhaps, as the house may not have had access to the main sewers. If anyone knows then please tell me :)

Many thanks to notowennewitt for looking after Derelict Sunday at the moment.