Here we are with the difintive answer from the UKHO:
Your query has been passed to me for attention.
You are quite correct in assuming that the direction of the magenta light flare is merely for the convenience of the cartographer. It is also for the benefit of the chart user, as it ensures that as little text etc. as possible, is obscured.
However, the exception to this is with Leading Lights, Lights in Line and Direction Lights. When these lights are charted, the magenta light flare lies in the direction of the pecked/solid lines. Ref 5011 (int 1) Edition 2 - 1998 (the symbols and abbreviations used on Admiralty Charts book) IP 20.1; 20.2;20.3; 21.
I hope this has assisted you with your query.
Please do not hesitate to contact this Office if you need further assistance.
Mrs Lynn Morley
Deputy Lights List Officer
Lights List Office
Tel: +44(0)1823 337900 3639#
Fax: +44(0)1823 334752
Good answer, not too long to get it either. So now you all know.
Keith (a leading light) Hart