An NSL sign with lamp posts should have repeaters at intervals:
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(2) Subject to paragraphs (3) and (4), signs to which this paragraph applies shall be placed at regular intervals along a road which is subject to a restriction, requirement, prohibition or speed limit which can be indicated by the signs.
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Note that the requirement to provide repeater signs to indicate that a road is subject to the national speed limit only applies if the road has a system of street lights (lamps not more than 200 yards apart). In this case the sign used is the 'derestriction' sign, known in TSRGD as diagram 671. The purpose of providing signs in these circumstances is to advise drivers that the road is not subject to a 30mph speed limit, which a system of street lighting normally implies.
The repeater signs are a requirement but the distance is variable. They are a requirement in areas subject to a speed other than 30 in an area lit by street lamps.
http://www.opsi.gov.uk/si/si2002/20023113.htmI do not recommend reading the above...if only because it is confusing.
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(4) The sign shown in diagram 670 (except when displayed on a variable message sign in the manner mentioned in regulation 58(7)(b)) shall not be placed along -
(a) a road on which there is provided a system of carriageway lighting furnished by lamps lit by electricity placed not more than 183 metres apart in England and Wales or not more than 185 metres apart in Scotland and which is subject to a speed limit of 30 mph; or
(b) a motorway on which a national speed limit is in force.