HMRC have updated their guidance on the rules for carrying forward the unused pension savings annual allowance. A calculator can be found on HMRC’s website, here: http://www.hmrc.gov.uk/tools/pension-allowance/
For most taxpayers the maximum amount of pension savings that qualifies for tax relief each tax year is £40,000. It is possible to increase this amount by utilising unused relief brought forward from the previous 3 tax years, provided the individual was a member of a pension scheme that year.
The brought forward relief from the earliest year is utilised before later years. Thus, for the current tax year 2018/19 the unused relief from 2015/16 may be utilised in addition to the current year relief, followed by 2016/17 and then 2017/18.