Can't trace the previous owner - no one listed at the address on the V5 (which in itself is a little worrying). I spoke to Lexus Bristol who told me that they don't have the technology to do it, but gave me the name of a Locksmith who they send people in a similar situation to. He apparently has all the gear to clone they keys, and the parts bloke reckoned it could be done off the sub key. He said that if not, it would cost £2000 for a new ECU, lock set, keys and labour!
The locksmith is closed until Monday, but if he can do it, I'll post back here with his details, so anyone else with the same problem can get it sorted.
Simon
(Oh, and sorry for repeating the topic, I did three different keyword searches on "key" and turned up nothing, then found a load of posts by just trawling through the board!)