A loud, regular 2 note call, like 'wok WOK'. It sounds like a throaty goose , maybe like a cross between a crow and a goose , but the squawk is short and curtailed, not like a drawn out 'caw ', and it is getting around a bit, I keep hearing it from different directions , near and far.
I live in S London, there are lots of big trees out the back, and a big bramble patch.