Re: A Non - Lawyer Writes...
"But that's perhaps the UK, where we don't have the 'poison tree' doctrine the US does"
The UK is pretty much the ONLY member of the EU (and the commonwealth) which has resisted adopting this doctrine - which is rather telling all by itself.
Of course a Spanish finding that "Fruit of the poison tree" material was used for the extradition proceedings in the UK, would pretty much destroy any hope of a trial in the USA.
(and the fact that the UK blocked Spanish investigators, WOULD be played up on heavily in a USA court)