He's been on his last legs for a while, so it's no surprise. He'd been on thyroid medication for over a year and was actually only given a few weeks to live back at easter due to a lump they found in his belly.
This week we noticed that one of his back legs was swollen and he was given anti inflammatory meds. The same day we noticed his front leg was swelling. We took him back and asked if it could be his kidneys shutting down. it could be, or it could just be age and poor circulation was the diagnosis. Take him home, carry on with the tablets.
Today Dh calls me (im at our caravan with dd2) to tell me he's waiting til the vets opens to take him again as the cat seems unhappy. Then I got another call to say that although he'd decided to take the cat earlier as an emergency, that when he went to Bruce to collect him from where he was moping in the garden that he'd taken a few laboured breaths and then stopped breathing. 
We've had him since he was 6 weeks old, back in 1995. Dd doesn't seem too bothered, but I'm sad.