Hi S,
You cannot 'backdate' a pension contribution made now to an earlier year. You used to be able to up until about a decade ago. The only thing you can backdate now is a charitable donation, in some circumstances.
What you MAY be able to do, which is of some potential benefit, is to reduce your 2016/17 payments on account, which are generally due on 31/01/17 and 31/07/17 by reference to a projected reduction in liability, by virtue of pension contributions you expect to make by 5 April 2017 that will potentially reduce your exposure to Higher Rate tax for 2016/17.
There is a model you can use at
http://taxationweb.azurewebsites.net/Pe ... debug=true
to project how pension contributions will affect your position for the current or indeed future years. Do please note, however, that you will suffer interest on any amounts you have cut down, that ultimately prove to be due.
Kind regards,
Lambs