An odd sighting in Central London

Hi all, apologies no media to share, but twice today I've seen a bird I can't place in Central London. I get into Euston Station in the mornings, and work on Russell Square. Walking from the station this morning I saw a quarter of birds fly overhead. I would estimate size to be approximate to a feral pigeon, maybe a tad smaller. They flew with a halting sort of swooping gait. As they flew over they were calling, and it was a rasping screech (best thing way I can describe it) every few seconds. Definitely not 'birdsong'.

Just now going out for lunch i saw one of the same birds in Russell Square. It sat high in a tree, with that same rasping call, and then flew directly overhead. 

The size and shape make me think of either a jay or GS Woodpecker, but the call doesn't match for the latter, and I can't imagine 4 jays coming into central London. I think colour wise it was a mix of black, white and cream, but I can't bee 100% sure. Any ideas?