TheOddOne, sorry but that's plain wrong!
Firstly, you haven't correctly covered the equipment mandated by Schedule 4 of the ANO for non-public-transport flight OCAS.
Yes, you're quite right, I do apologise. No compass required, but a turn & slip instead. Altimeter I was right about...
(2) Aeroplanes
(a) flying for purposes other than public transport; and
A(1) and (2) and B(1), (2), (3), (4), (5) and (6)
(ii) when flying under Instrument Flight Rules;
(aa) outside controlled airspace - D
A and B are about equipment, seat belts etc
D says
Scale D
(1) In the case of a helicopter or gyroplane, a slip indicator.
(2) In the case of any other flying machine either—
(a) a turn indicator and a slip indicator; or
(b) a gyroscopic bank and pitch indicator and a gyroscopic direction indicator.
(3) A sensitive pressure altimeter adjustable for any sea level barometric pressure which the weather report or forecasts available to the commander of the aircraft indicate is likely to be encountered during the intended flight.
I can't see any other requirement for
IFR flight in the UK in VMC OCAS.
I have several friends with aircraft with no electrical systems who fly non-radio who sometimes fly in accordance with IFR VMC OCAS below 3000'.
TOO