"Alexandrine Parakeets
These are much less frequently encountered in England, although some sightings may have been lost because it is easy to confuse identification with the more common Ring-Necked. Flocks of Ring-Necked parakeets sometimes comprise an Alexandrine too. Alexandrines are essentially natives of India but stretch in a band from Afghanistan through to Indochina.
Male/Female and adult/juvenile variations as well as mutations and perhaps the effect of differing diets all serve to confuse identification but the Alexandrine is, at 60cm a significantly bigger bird than the 42cm Ring-Necked. In addition, the Alexandrine has a bigger bill with both upper and lower halves coloured red. Alexandrines have a red shoulder patch which the Ring-Necked parakeets do not - but in the field you may not be able to see this."
Possibly - they have been known to interbreed with other parakeets and the one you have spotted may well be a hybrid ...
