Severn Trent make it quite plain on their website that it's down to me if the service pipe fails -
Your responsibility
By law, the owner is responsible for maintaining the pipes running under their property, garden or driveway that supply their property. They are also responsible for the pipes within their property. This means that if there is a leak from any pipes within the property boundary, the responsibility for making a repair falls to the property owner. In some circumstances, responsibility may extend beyond the property boundary.
Our responsibility
We are responsible for maintaining the pipes which run outside your property boundary...
I don't know why this should be; why is it so different to other utilities -phone, electric and gas?