aha. Found it! Here is the authorities word on unpublished approach minima...
From UK AIP AD 1.1.18
11 Aerodromes without published Instrument Approach Procedures
For an aircraft landing at an aerodrome without an instrument approach procedure either:
a A descent should be made in VMC until in visual contact with the ground, then fly to the destination;
b An IAP at a nearby aerodrome should be flown and proceed as in (a); or
c If neither (a) nor (b) is possible, first obtain an accurate fix and then descend not lower than 1000 ft above the highest
obstacle within 5 nm (8 km) of the aircraft. If visual contact (as at (a) above) has not been established at this height, the
aircraft should divert to a suitable alternate with a published instrument approach procedure.
Again note - 1000' above highest obstacle - so even over the sea - as I said - you are looking at 1300' AGL (well AMSL if you are talking about let down over the sea but lets face it that means you must be nowhere near land - so if you go anywhere near land you are actually looking at 1400' AMSL as a minimum)...