Just to pick up on a point Simon mentioned in the IS section that he had on his GS. For anyone who has this problem
I too had the squeak and the dealer I use Lexus Twickenham (who whilst not the most local for me are very good at problem solving), replaced the windscreen scuttle plate which is actually under the bonnet. The sound went away but a year later has now just started to come back. The technician explained that the clips that hold the part in place work loose producing the noise.
As for the re-occurrence I agreed with the Technician that we would wait until it gets worse, and then they will replace and/or pad the clips at my convenience.

Regards
Keith