We've just had to fork out over £1,000 as a pipe under one of 8 of a row of terraced houses had a leak, and the whole lot had to be replaced. We had to foot the bill between the 8 houses- the water pipes under your house and garden are your property and not the water board's.
You can pay only £1 per month pipe insurance so you are completely covered if this ever happens to you - please can I encourage you to look into whether this is applicable to you if you are a homeowner, as it is SO cheap to insure but SO expensive should this happen. We are in a victorian terraced house and our plumbers said this was happening more and more.