He was 11 years old, the vet said that he had been poisoned by antifreeze 
He came in this morning after a night on the tiles with his brother and promptly went to sleep (normal). However, he started vomiting on the sofa. I feel awful now but I lifted him out on to the patio while he was being sick and his back legs just gave way underneath him. He stumbled back in to the house completely disoriented and collapsed at my feet.
He then struggled out of the living room and went to sleep on our bed.
Each time I went to check on him he didn't flinch. I had to physically touch him to get a lame mew.
This is a cat who is afraid of his own shadow and stayed on the bed while strangers were viewing our house and traipsing through the bedroom.
I took him straight to the vet, and as he could find no obvious sign of injury he suspected poisoning.
I got a call later today to say that he had passed away and there was antifreeze in his system. My poor tiny little boy cat.
I can't help but get upset thinking about his brother that he's left - they were worst enemies but best of friends.
He also loved 9 month old DS regardless of the tail pulling and fur grabbing.
Sorry for such a depressing post.