LONDON, Jan 21 (Reuters) – North Sea Forties crude was offered lower on a delivered basis on Tuesday, reflecting ample supplies and a lack of demand. PLATTS WINDOW * No deals * Forties: Mercuria offered a Feb. 2-6 cargo on a cost, insurance and freight (CIF) Rotterdam basis at dated Brent plus 90 cents. * An industry source said the offer translated to about dated Brent minus 28 cents on a free on board (FOB) basis, meaning Forties has weakened from Monday’s value of dated Brent plus 8 cents. (Reporting by Alex Lawler; Editing by Jan Harvey)