LONDON (Reuters) - Britain's public spending watchdog on Friday questioned a government estimate of 24 billion pounds as the cost of tax relief to decommission oil and gas infrastructure.As one of the oldest and most mature oil basins, Britain's North Sea represents the world's biggest decommissioning project, with the Oil and Gas Authority estimating operators will need to spend around 60 billion pounds.The government has given oil and gas operators different kinds of tax relief designed to..