Really helpful thanks Daryl. For those in the UK, I also found that you can use your Unique Taxpayer Reference (UTR) as the Foriegn Tax Identifying Number - for those that already complete a tax self assessment. Otherwise a National Insurance (NI) Number will work.

So all looks good for now but can't help but feel that I will have no idea what to do if the US government needs me to do somehting further! Hoping there's no need to file any annual returns with the US from this point forward.