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I am guessing, but I would think that Council will need to charge staff out as 2x to 3x salary (to cover Employer's National Insurance, Pension, etc. the overheads of Office space and Admin staff that manage HR, Wages, and Management).So £18,000 would get the services of someone who's salary was £9,000 or maybe even only someone paid £6,000.No idea what the salary is of such clerical folk, but if £20,000 p.a. and they work, say, 200 days a year (after sickness and holiday) then you'd get 90 - 60 days p.a., and if the salary is £30,000 then 60 - 40 days p.a.No idea if that is any good as a starting point though?
The likelihood is that you are being subsidised by the Council (tax payers) and each year they will be trying to reduce that subsidy. As to whether you are getting value for money then that is another question. If you want to reduce your costs then the answer is self management and you may wish to consider this option.
Personally I think it depends on how big your site is and how many volunteers you might have if you are thinking of getting self-management sorted. Self-management can be hard if you are on a small site. Quite often most of the jobs end up being done by one person. Perhaps you could have joint self-management with other sites to help spread the load?