Great White Egret?

This morning I saw what looked like a white heron in the River Blackwater between Farnborough North and Frimley.  It was about the size of a grey heron but with pure white plumage and a black beak and black legs.  As I approached, it flew up from the river and disappeared behind some trees.  A bit further along the path I saw it again in the river and this time, when it flew up, it looped back over my head, allowing me a clear view.  It flew with its head tucked back over its neck, like grey herons do, and had its legs trailing out behind.  My reference books suggest that this was some form of egret, probably a great white egret (Ardea alba - sometimes  referred to by the older name Egretta alba).  However, the beaks of great white egrets are usually yellow and mine appeared black, so there may be room for doubt.  Anyhow, I don't remember seeing anything like it before.

Update on Sat 2008-03-01 by Tristram Brelstaff

I saw this bird again in the same place on the following Monday morning (February 25).  I only got a brief glimpse before it flew off but it definitely looked a bit smaller than a fully-grown grey heron.  I now think that it was a little egret (Egretta garzetta).  Little egrets are known in this area.  Here is a photo of a pair at Dinton Pastures near Reading.