The original quality, ie Lucas hence the L part of the part number are better than the repro versions now available in white boxes. Consider replacing just the chrome/glass outer if the base-plate is still good. I got mine on ebay (Rover P4 in good used condition for £10. Try Automart at Bealieu this weekend. www.beaulieu.co.uk If you find a P grille or any Phase 1 badges we'll be fighting in the field for them. A suitable thickness acetate can be used for the lenses if necessary as a home repair.
Indeed it comes from a Rover P4. I got mine from my brother who once owned a P4, but sold it in bits. The number plate/reversing light somehow wasn't sold. On my Phase 1, only the chromed part is outside the boot lid. The part which holds the lamps is inside, the bootlid is clamped between. If i'm right, on Warwicks the complete light is mounted on the outside of the bootlid. It fits in the modified bootlid shape. They turn up at Ebay quite often.