Many thanks to everyone who sent messages of condolences. My family and I are truly touched.
We are still in a state of shock. It is the suddenness in the way he went. I now understand that cancer of the spleen is relatively common amongst older male dogs. If they can stop the bleeding and remove the spleen, there is often a good outcome. BUT because any operation is fraught with danger (including blood clots), vets prefer to wait until the tumour has burst. In Joss's case, they did stop the bleeding and successfully remove the spleen, it was the blood clot which was the problem.
One tends to think once the operation is over, all will be well. However, this is why he was in the hospital. These things happen -- to humans as well as dogs. It is why for example humans wear surgical stockings...
So we are slowly adjusting to our loss. He was a big part of the family -- always wanting to play fetch with anyone -- capable of raiding bins in search of paper to fetch, trying to communicate through any squeaky toy and adapt to sing. He also loved to run, had incredible stamina and loved being on the go. He was highly intelligent in a manic border collie way and would attempt to anticipate your next move, particularly when out on a walk. He just was.