tigerpussycats, One of my grandfathers was a cockney and in post 3, it is one of those, but not Hayling or canvey. Try googling the other two and one of them will give you the answer.
There is a Mud Island in Hornchurch, but this is not the one used in the cockney's language. There is a book by William Matthews which will give you the right answer :)
There is a Mud Pie Island in Mississippi -
hence the famous Mississippi Mud Pie - which tastes much nicer than it sounds!
I live not far from Hornchurch and has never heard of it there. mag