"In the Meo info the structural spill point is mapped at 4325 m - SS ( which I assume is Sea Surface.- can someone verify this pls ) ."
SS – Subsea, as in a datum of depth, e.g. TVDSS (True Vertical Depth Subsea)
From wiki'. So your guess of sea surface was on the mark.
Looking forward to a big week ahead.
Thanks BM for your input.