Have you checked your Financial Regulations? The answer should be in there. Our reads:
1.1. No expenditure may be authorised that will exceed the amount provided in the revenue budget for that class of expenditure other than by resolution of the Council, or duly delegated committee. During the budget year and with the approval of Council having considered fully the implications for public services, unspent and available amounts may be moved to other budget headings or to an earmarked reserve as appropriate (‘virement’).
All Councillor are responsible for finances and approval of anything financial should be by full council or a duly delegated finance committee. Neither the Clerk or the Mayor can make unilateral decisions. Sorry, but I can't help with actual legislation.