Is it a mast?

No, it's actually the top of the central support for the new foot/cycle bridge in York, which I have blipped previously.

I had actually gone in search of the swan family who I was pleased to find flourishing. There are actually six cygnets not just the two which I had seen previously. They are now almost fully grown. Unfortunately, I could not get close enough for a good view but you can see the result here.