Tip: if you're stuck for a replacement copper washer you can always re-use the old one by annealing it. Just hold the washer with a suitable insulated piece of wire over a flame, such as the hob, for enough time for it to become red hot all over. Drop it into a glass of cold water and clean off the black deposit. The process softens the copper and renders the washer re-usable. My mechanic anneals new copper washers also, when possible.