Does using an extension lead use less or more electricity than if the plugs were in lots of different sockets?

slightly less since it adds resistance but you get a lower voltage at the end of the cord. Some devices will draw more amps to compensate others like light bulbs will just be dimmer...

Actually about the same since if you are using an extension cord it is because there is not a socket close-by. If you add a socket close by you must bring a wire to it and assuming the wire is the same size the voltage drop will be the same. If you add heavier wire then the multi socket will be better but the cost of installing the socket must be figured in also.

There are many factors making it situation dependent.

THE LONGER EXTENSION CORD ( LIKE 200 FEET ) IS USED TO PASS HEAVY CURRENT WITH MANY LOADS AT ITS END, THE MORE ELECTRICITY IS WASTE AND TURNING INTO HEAT ALONE THE LONG CORD. BECAUSE EXTREMELY LONG CORD WITH THIN GAUGE WIRE HAS LARGER WIRE RESISTANCE.

IF YOU TOUCH THE EXTENSION CORD WHILE IT IS UNDER FULL LOAD LONG ENOUGH AND YOU FEEL IT QUITE WARM, IT WASTES MORE ELECTRICITY AND TURNS IT INTO USELESS HEAT ( PERHAPS GOOD TO WARM UP THE ROOM IN WINTER, HA HA ! ).