jim_in_sussex wrote:

Yes thanks, but it's the words 'clipped direct' that throw me.
Loosely resting on plasterboard seems a long way from being clipped
every 9 inches to a timber joist.

The full description of Reference Method 1 (clipped direct) in Appendix
4 of BS 7671 is "sheathed cables clipped direct to or lying on a
non-metallic surface."

But I suppose resting on cold p'board might be as good.

You suppose correctly.

Returning to the original question, the 50mm separation requirement only
applies where a cable passes through a timber joist. At other points
along the run the requirement is only that it "shall be run in such a
position that it is not liable to be damaged by contact with the floor
or the ceiling or their fixings" - see Reg. 522-06-05.

--
Andy