Having received a rebate from HMRC for Employers Ni contributions after completion of my self-assessment submission, I posted news of this above. It is with some regret that I must advise that many months later I had a demand for under-payment of tax and a notice to pay immediately or suffer delay penalties. I duly paid up but continued to plead a case to no avail. The good news was that as I had not tried to conceal information they would not impose a fine!
So, although drawing state pension for over 20 years, The payment is still liable to Employers NI. Sorry, folks.