Can you please tell me what washer I need to stop the leak from the nut where the single lever mixer tap is connected to the pillar legs on my hot water side.

Pictures, or make and model numbers would be very helpful, and do you mean the sink, or tub? Typically they will all involve the cartridge, but a bit more information please.

Yes, a picture would be nice. Most of the plumbers here are USA and we use different terminology. Few if any speak the Queen's English.