Very interesting, I take it the map is of England in 1000AD, if so the islands are very interesting, I didn't realise there were many off the east coast. Presumably the sea level was much lower then.
The Fens and the Wash can be very confusing to portray. Either way, the islands were supposedly part of Lincolescire / Lincolnshire after counties essentially replaced the Heptarchy system by the end of the century.