Hi Alan,
No experience in this myself at all.
But if you have a look here:
http://www.equipped.com/1199ditch.htm
You can read some interesting stuff. The PA28 they flew over was 10% overweight and as they state 'allowable' I take it that someone can give you dispensation for overgross operation.
I believe that Andy Bruce at FarNorth is also a fond of information he is at Wick. He is a very friendly and helpful figure and am pretty sure that he would be willing to give further advice. He was certainly a great help when we went there last year on our way to Iceland.
Have fun.
FD