The Mediterranean grocers, real name East Dulwich Supermarket aka "Mateys", sells lots of things that the deli does but at a better price e.g. olive oils, lots of different types of rice and pasta, obscure herbs, spices and sauces, turkish delight etc. I've noticed that when the Deli has a pancetta display so does Mateys but his are half the price. They sell things that you would buy in a super market abroad such as a packet of risotto rice where as over the road you'll probably find Authentic Hand Picked Italian Rissottorini rice in a cloth bag with hemp stitching and a bow, similar contents, different price. I wouldn't say there was a clear line between them (Lordship Lane excluded) but there is an overlap.