Those look more like Australian green parrots rather than parakeets. Mum's parakeet tied the other week after a few weeks of illness after a stroke (we think).
They're ring-necked parakeets (also known as rose-ringed parakeets), from India. The story is that they're descended from Jimmi Hendrix's, released after he died - and there are over 30 thousand of them across London...
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Here are a couple of good links about them!
http://animaltourism.com/news/2011/05/20/london-parakeet
http://www.projectparakeet.co.uk/
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