Now, the CAA have told me "this is an old reference" So...where is the proper up-to-date UK reference for this rule?
It is shocking if you are quoting the CAA correctly. This is an important flight planning issue. e.g.
https://www.legislation.gov.uk/uksi/2005 Rules of the Air Regulations
SCHEDULE 1
SECTION 3LOW FLYING RULE
Low flying prohibitions
5.—(1) Subject to paragraph (2), an aircraft shall comply with the low flying prohibitions in paragraph (3) unless exempted by rule 6.
(2) If an aircraft is flying in circumstances such that more than one of the low flying prohibitions apply, it shall fly at the greatest height required by any of the applicable prohibitions.
(3) The low flying prohibitions are as follows—
(a)Failure of power unit
An aircraft shall not be flown below such height as would enable it to make an emergency landing without causing danger to persons or property on the surface in the event of a power unit failure.
(b)The 500 feet rule
Except with the written permission of the CAA, an aircraft shall not be flown closer than 500 feet to any person, vessel, vehicle or structure.
(c)The 1,000 feet rule
Except with the written permission of the CAA, an aircraft flying over a congested area of a city town or settlement shall not fly below a height of 1,000 feet above the highest fixed obstacle within a horizontal radius of 600 metres of the aircraft.
(d)The land clear rule
An aircraft flying over a ........ etc