Just to add a bit more detail, I was in the hide watching the gull roost in the very pleasant late afternoon sunlight when the whole flock took to the air at once. My initial thought was that maybe there was a raptor around--a Peregrine perhaps.
Then I saw a large dark bird heading towards me about 100-150 feet up and as it turned it was obvious that it was a GREAT SKUA, being all warm brown with large white flashes at the base of the primaries. It headed off towards the far side in pursuit of a LBB Gull, which it harried in typical fashion over the trees for a short while, but gave up, returned towards the hide, then back across to the far side when I managed to take some rather poor photos (see above). It dropped down briefly in front of the reeds and at this point I unfortunately lost it whilst trying to find it again in my camera viewfinder and my guess is that it headed off north.
The gulls returned to roost on the water and all was peaceful when I left to take a quick look from the gate to see if perhaps it was on the water in the hidden arm of the lake, but no luck.
Phil Tizzard