The complete rear light units are still available new via mail order from the companies listed below. They come supplied with the rubber gasket to go between the unit and the bodywork and all four bulbs. The prices below are for two units including VAT and postage within the UK (correct at 1st March 2007).
The only difference between the new lenses and the originals is that the new versions have a clear section to illuminate the number plate. On a Portafold or Ansfold this isn't applicable as they have a separate number plate light so all it does is light up the GRP below the rear lights. You can avoid this by buying a pack of light lens tape (available from your local motor factors) and sticking the tape on the inside of the clear part of the lens.
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