Left US after 2 year secondment 4 years ago. Cashed 401K and paid US tax in 2012. I have no experience of the US tax system and am being told I will need to submit a form 1040NR. The terms on the form and incomprehensible to me. Is there an easy way to do this?
If the terms are incomprehensible, then no, there's no easy way. You have to pay someone to help you (or if you're lucky enough to have a US tax professional in your circle of friends - get some free help).
You probably paid 20% tax withholding on your withdrawal. You're likely to need to pay more.
You can (and should) ask your employer to support you on this, if you were sent to the US by them, and also see if the tax treaty between your country (UK?) and the US provides you benefits, which it might.