Neil Herron's site might be helpful:
http://www.parkingappeals.co.uk
Section 18
The Local Authorities' Traffic Orders (Procedure) (England and Wales) Regulations 1996
Traffic signs Regulations & General Directions 2002
Illumination of signs
18. - (1) Subject to paragraph (2), every sign shown in a diagram whose number is indicated in column (2) of an item in Schedule 17 shall be illuminated in the manner and at the times specified in column (3) of that item.
(2) Where a sign shown in a diagram whose number is indicated in column (2) of an item in Schedule 17 is placed for the purpose of conveying to vehicular traffic a warning, information, prohibition, restriction or requirement which applies only at certain times, the sign need only be illuminated, and in the case of the sign shown in diagram 776 shall only be illuminated, in accordance with that Schedule at those times.
(3) Where a sign shown in a diagram whose number is indicated in column (2) of an item in Schedule 17 is illuminated by a means of external lighting, then that means of lighting -
(a) shall be fitted to the sign or the structure on which the sign is mounted or which is otherwise specially provided; or
(b) if the sign is mounted on a bridge, tunnel or similar structure over a road or is provided temporarily at road works, the means of lighting may be mounted in any other manner such as to illuminate the face of the sign effectively.
Illumination of signs - further provisions
19. - (1) Nothing in this regulation shall apply to the signs shown in diagrams 560, 561, 776 and 781.
(2) Subject to the provisions of regulation 18 and paragraph (1), any sign shown in a diagram in Schedules 1 to 5 and 7, Part II of Schedule 10 and Schedule 12 -
(a) when placed in consequence of the execution of road works must, and
(b) in other situations may,
be reflectorised in accordance with the following provisions of this regulation.
(3) Subject to paragraph (4), where retroreflecting material is used on any part of a sign shown in a diagram, all other parts of that sign shall also be reflectorised.
(4) No retroreflecting material shall be applied to -
(a) any part of a sign coloured black;
(b) that part of the sign shown in diagram 7031 which is coloured fluorescent yellow, unless the retroreflecting material is applied to that part in horizontal strips with a gap between each strip, or unless the retroreflecting material is itself also fluorescent;
(c) those parts of the signs shown in diagrams 2505, 2505.1, 2506, 2507, 2508, 2509.1, 2510, 2511, 2512, and 2513 which give information about the availability of parking places when that information is conveyed by means of a legend which is internally illuminated or formed of light-emitting characters,
and in this paragraph the word "part", in relation to a sign, means any part of that sign which is uniformly coloured and bounded by parts of a different colour.
Illumination of plates
20. - (1) Where a plate is placed in combination with a sign shown in a diagram in Schedules 1 to 5 or 12, and that sign is illuminated in accordance with regulation 18, the plate shall, subject to paragraph (2), be illuminated by the same means as the sign.
(2) Paragraph (1) shall not apply where the means of lighting provided for the illumination of the sign adequately illuminates the plate.
Illumination of signs shown in diagrams 560 and 561
21. - (1) The signs shown in diagrams 560 and 561 shall not be illuminated by the fitting of a means of internal or external lighting.
(2) A sign shown in a diagram whose number appears in column (2) of an item in the Table and having the dimension specified in column (3) of that item shall be illuminated by either of the methods prescribed by paragraph (3) which are shown in column (4) of the item, and by no other method.