Mine's pretty much the same -looking at 2 or 3 recipes I think you can more or less wing it as the amount of sugar & lemons varies considerably between them.
Anyway :
9 large lemons
4oz caster suger
2.5pts water.
Pare lemons and place rind and suger into a bowl -boil the water and pour over until suger dissolves, cover and leave to stand for 30 mins.
Squeeze and strain juice (ideally you will get 3/4 of a pint), pour the above syrup, strained, into a jug add the juice and refrigerate until chilled (2 hours+).
Serve.
Personally, I would add a large sprig of mint to the jug. I'll have to get my mum's recipe -it tasted differently to this, less intense and more thirst quenching.
Jeremy