This place is basically a live scrap yard which specialises in scrapping railway rolling stock. As someone who has travelled on the trains they are currently scrapping all my life, I had to go there to see them in their final resting place.
The part of the site we were in was the final home to around 20 cars of Ex-Metropolitan Line A60/A62 stock which have all been withdrawn in the last few months, some as recently as a week before our visit (as specified in the trains' defect log book).
Interestingly there are also some Ex-Victoria Line 1967 stock trains there, which all had a notice stuck inside them which read 'This car is to be re-used DO NOT SCRAP' and also a contact name and number. I have heard that a few are being stored at Eastleigh, however it isn't clear why they are storing them at Rotherham too.
I'll leave the photos to tell the rest of the story.