Circuit's Odd Friendship

Here are 2 pics of the boy-cats because they are lucky to be alive. When I went down to whistle them indoors last night, they didn't come within 2 mins as usual so I went looking around.
The neighbourhood fox loped by, about 6 metres away, with CIRCUIT alongside. The pair of them proceeded to sniff around and seemed entirely at ease together. The fox must've sensed me as it suddenly took off fast, and Circuit trotted over. Panzer crept out of the bushes; he must've been following them, but in a PROPER cat-cautious way.

NB A friend of mine in Kent has had TWO of his cats killed by local foxes.

I suppose that, short of putting them under house arrest, I can only keep putting new flea collars on them.
Trust Circuit, the Nutter-Cat.