Many Thanks David,
I had a sharp intake of breath when I read your reply, but I'm better now. I think I can see where you're coming from and the worse case won't apply to me.
I presume the 10% you mention is an early withdrawal penalty, but I turn 60 this year so wont affect me (I already cashed in an IRA as soon as I could, in case the turbulent market turned sour just as I was about to exit the US). I'm not remotely close to paying top rate tax, even though this year will be my biggest ever gross income by some margin (but never to be repeated, which is why I'm focussed on not screwing up!). I expect my taxable income after deductions to be about 180, including that IRA, but excluding the pension, so I'll top out in the 32% bracket.
So the point of my question was to understand how the income numbers get added up (or if they do) in a dual status return, because it seemed to me that I'd be better off waiting until I was a non-resident in December (or Jan 2020) to take the lump sum pension, since it would be taxed by itself at a lower rate. Then I got to thinking it seemed too good to be true that you really get to file the equivalent of 2 part-year returns, since the total tax appears to be lower (to my way of thinking) than if I was a resident all year. But after reading the IRS instructions, I was still unclear about how the dual status return actually works, hence my question.
As I understand it, in 2020 when I'm back in England, I will have to file a non-resident return for 2019, with a resident return attached to it. But does the income from the resident part get added to the gross in the non-resident return, to get an overall gross in the 1040NR? Or are the resident and non-resident parts treated absolutely separately and the final tax bill is the sum of the tax computed for each?