Well it seems that HMRC assume all state pensions are paid weekly. In my case it is four weekly. As I was paid 17th March (in arrears) I am being taxed on money I had not received. However, I still cannot get the exact figure stated by HMRC even if I reduce it to a daily rate and work out the number of days I was paid in 2016/17 (I was only retired for two months). So it's another phone call or two next week. What a way to complicate things!
HMRC are correct insomuch as the State Pension, as are all benefits, classed as weekly entitlements no matter whether they're paid weekly, fortnightly, monthly or quarterly.
Below is the official HMRC guide for recording the State Pension on the SA100 tax return form.
State pensions.You are taxed on the 'full amount' of state pension you were entitled for the year to 5 April 2017, which is the total of your weekly entitlements. As a guide, to calculate the total of your weekly entitlements for a full year, if you were paid:
Weekly
Add up the 52 weekly amounts as shown on your bank statement or building society passbook (if you were paid by direct debit)
4-Weekly
Multiply your 4-weekly amount by 13
Quarterly
Multiply the quarterly amount by 4
HMRC collect income tax based on what a person was due to receive in a tax year whether or not they did receive the payment in that tax year.
Many small businesses fall foul of this rule by not declaring invoices issued where payment has not yet been made.
E.G. If your trading year ended on the 31st March 2017 & you issue an invoice for £1,000 on that date but didn't get paid until the 15th May 2017, that £1,000 & the tax due has to be included in the 2017 tax year, not the 2018 tax year.