Rob, These draught marks are generally dust that gets drawn up through floors and gets deposited around edge of carpets via skirting boards. Can find them as lines under doors, especially airing cupboards and front doors. Worst cases are rows of dots coming out from skirtings........caused by knee kicker piercing membrane in carpet during fitting. This is sometimes quite pronounced and harder to remove, although it could have been avoided in the first place.
I always use Prochems Citrus Gel or Ashbys Anti Grease with great success but warn client that results may not be 100% as marks are from backing. Wetting through to backing carries risks of browning so CAUTION. Marks will reappear eventually but advise that problem areas are THOUROUGHLY vacuumed monthly to keep at bay.