Well I visited again today and met a fellow who was alongside me at the time, interesting his pictures are just as bad - extreme heat haze - bright sunshine.
Also met the local RSPB warden who was able to confirm our sighting of a Purple Heron in the exact spot that it had been reported on numerous occasions.
alanc posted a picture of the same bird in the almost exact same location - LL on the Isle of Wight. Thankyou to all who have taken the time to post, it has been very informative - thanks again !
Thanks especially to John Cantelo for his informative picture and to Alan for digging out the other picture, both helping with confident identification.
With the better picture, I think its safe to call this bird a 2 cy (?). Here is an older immature Purple Heron for comparison, where I got help regarding ageing from Christian Stohl and Julia Bayer (Schwetzinger Wiesen, SW-Germany, 09.04.2011)