Thanks everyone for your lovely condolences.
All of our cats are indoor cats but occasionally Messr. Boo Boo was allowed a walk in the garden and a chew on some grass as we knew he wouldn't take off.
He was the most brilliant cat (and we've had 10 cats - 8 at once - now down to 3 black cats! Yes, yes, I know, not by design I can assure you, we're just suckers!) never did the wrong thing, didn't pick on any of the others, loved his routine, was very intuitive. He was a British Blue Shorthair (as was Bazzi) - a fantastic breed - and I'd dearly love another but feel I would be disappointed if it didn't have the disposition of Messr. Boo Boo. We can't quite remember how he ended up with a French salutation given he was British!
On Sunday night he insisted on coming up onto the bed, looked at my partner and I, meowed as if to say 'clearly I am going to have to be the adult here and make the decision?' and we just knew then it was time.
I held him through till his last breathe. We buried him next to Bazzi, in the garden, along with the others. We will miss him forever.