There is at least 1 regularly viewable Iceland gull, and possibly more in the Thames at the moment. Two seperate birds a 2nd winter and 1st winter have been observed following ships up the river (generally larger high sterned vessels) on consecutive Saturdays.
The 1st winter on Saturday appeared to feed from the wake of the ship. After feeding it appeared to be prompted by a vessel heading downstream, flew into the wake and then flew strongly NE landward towards Coryton.
The Gulls were viewed from East Tilbury at the Coalhouse Fort radar tower, or from the sea wall/grass bank downstream of the Fort.
Kittiwake numbers were down to 8 on Saturday from 87 the Saturday before.
This site and associated app http://www.marinetraffic.com/en/ is useful in seeing number and size of ships navigating the river.