By Mark Kleinman Mark Kleinman is Sky News’ City Editor and the man who gets the Square Mile talking in his weekly City A.M. column. Today, he looks at the UK’s altnet sector, a jilted housebuilder and Thames Water’s leaky boardroom. Britain’s fibre infrastructure proves hard to digest Time to hang up? That’s the way many investors feeling about Britain’s fibre broadband infrastructure, or altnet, sector. After ploughing billions of pounds into trying to conquer pockets of Britain in the last five years, few are holding out hope of a positive financial return. Take G Network, occupier of much …