Thanks for that Cam, it's not at all how I imagined! Just to be clear, this withholding applies even to money paid to non-US-citizens as long as the work is done for a US firm?

Does the non-US contractor have any US legal obligations if this tax isn't properly withheld? I'm quite certain I'm doing the right thing on my UK tax returns, but I'd hate to find I had trouble doing business in the US because of old unwithheld taxes...

I, and I'm sure many others, would like to know about share options for foreign nationals - again, I'm comfortable with the UK rules and regs, but is there also some US liability for foreigners taking share options?

Thanks again, your kindness is really heartening.