My headteacher has just told me he is taking away one of my three management points after poor results this year.
Your head may be acting rather prematurely. The pay review body is considering a recommendation from the rewards and incentive group set up by the DfES, employers and teacher associations (minus the NUT) for a new framework for responsibility payments. Any new arrangements will replace all allowances with only limited safeguards for existing holders. This is why all allowances granted since April have only been temporary, for a maximum of 12 months.
Assuming your points were awarded before April 2004, the teachers pay and conditions document clearly states that these points were awarded "for the period determined when the award was made, whether for a fixed period or while the teacher remains in the same post". There may be other ways of tackling poor performance, but this is not an acceptable one.