Thanks Peeps, interesting stuff, especially Biggles.

We are on a meter, all our surface and waste water goes down the same manhole into the sewerage system (has dome in all our houses)

One thing I did spot somewhere is that if you use a load of water you wouldn't normally use that doesnt go to the sewer (ie, filling a pond / pool), then you can claim a rebate for that amount.

We did have the builders in (not a euphemism!).
Last year, and they used a lot of water, I suppose that which went into mortar & plaster wont have gone to Sewerage at the very least.