Hi geoff
According to the Avifauna Purple backed is extremely rare in Spring, with very few records, and Chestnut-cheeked is alo rare, while White-shouldered is the more likely.
However the dark eye separates from White-shouldered and the extent of white on the wing looks to be too much for Chestnut-cheeked Starling.
In my opinion (and I quite happy to be corrected) this is indeed a Purple-backed Starling.
A nice record!
Cheers
Mike K
Mike KilburnVice Chairman, HKBWSChairman, Conservation Committee