Thanks everyone, I have made a small amount of progress...
I have located the 3rd fuse box (Driver's side, behind the bonnet release)
Unfortunately as the seats are electric I can't move them back to get access to the cabin fuse boxes !
More importantly, having spoken with a helpful Lexus dealer, the suspicion is now on the "Block Assembly Fusable Link".
I can't really see for myself if these have blown (it seems a strange design, I'm sure that there's a good reason for it...) I can't really get a photo with good focus, but it does look a bit like these are blown.
I have a local garage coming round to have a look to confirm. Hopefully if it is, they can order and replace, otherwise it looks like I'll have to get it towed to a Lexus dealer.