So finally figured it out:
Firstly, fill in the US W-8BEN Form online to allow you to trade in US Stocks
Then call IG - there is an over the phone questionnaire which you can do which allows access to some US ETFs (not all)
You are likely a 'retail client' like I was, you can consider upgrading to a professional client if you meet the criteria - IG can email you the link when you call up
Finally, open spread beeting / CFD accounts - even if you are not planning on using them right away.
Doing all these things, seem to allow me to trade US ETFs within a Share dealing account from the UK.