Ragman0408 Garrad

I have lived in the US for 24 years and am a US citizen. I retired at the end of last year and have a pension in the UK with Standard Life (2 annuities and 1 pension plan). I have been told that I will have to take it as a lump sum and will be taxed at 40% , taken out at source. As a US taxpayer will I be able to claim a refund on the tax paid and pay the tax due in the US under the UK US tax treaty? How do I go about this and what is the process and forms to be filled to enable this. I have been told different things and want to be sure of doing the process correctly to avoid being taxed twice.