Same happened to us (we've bought the house that Jack's dipsh1t a$$hole of a thick-tw@tted in-bred brother built!!!
Too many issues to list, but suffice to say that £15K extra and 6 yrs later, we've nearly undone all the hidden damage that we found. (solicitor, building society and all the people who you "employ" to sort this suddenly put their hands up and ran away!!
Does this make me sound bitter?
Anyhoo, had the back patio dug up. After going through 3 layers of patio slabs (yee haa, ride em cowboy!), the builder must have been wearing is lucky rabbit's foot. As the spade went in, a nice yellow pipe came up with the rubble
Luckly, undamaged. Looks like the bodgers had moved the gas meter to a different part of the house (probably without telling British Gas!)......
Again, this was literally below the patio slabs.
Great fun though, isn't it?
Good luck getting it sorted quickly