It'll be a change in tax code - my pension went down by £100 a month for the same reason....
Oh, why did this happen, because the pension paymasters applied the full personal allowance to my pension payments for the year, and the RAF had done the same to my pay for my last few months in service. As a result I had had the benefit of 2 personal allowances and had paid too little tax, and it was clawed back by changing my tax code.
The OP should have got a letter from the pension paymasters telling him his new overall pension for the year 15-16, which should have gone up by 1.2%, and how much this would equate to a month, which appears to have gone down, given the tax code he is currently operating under according to HMRC!