Section 137 (1) of the Local Government Act 1972 is a statutory power. It gives council's the power 'to incur expenditure for purposes not otherwise authorised.' It is only used when there is no other specific power available; it is a 'power of last resort', it must benefit some or all inhabitants, not an individual, nor can it be made retrospectively.
S137 was amended in s36 of the Local Government and Housing Act 1989. The amendment stipulates that expenditure and benefit must be balanced.