I keep getting stung for withholding tax on foreign shares I own, mostly US shares, even though I've filled in a W-8BEN form.

Reason?

For cost reasons shares are purchased in a pooled nominee account and so the IRS can't see that I'm the beneficial owner.

Anyway, for those in a similar situation, this article was useful to me:

Watch out for withholding tax

PS. Holding the shares in an ISA or SIPP makes sod all difference, the septics couldn't care less and take the tax at source.