Update : bugger!
I could easily replicate the fault for the garage.
Sadly there are absolutely not fault logs, misfire counts or anything at all in the computer to indicate a problem. The car has NEVER misfired, not once, according to the logs which they say are very accurate and trustworthy.
Tech one have been brilliant and have even called in a Lexus technician and tracked the service history of the car and that Lexus did indeed change all the spark plugs last year at the 6 years service.
The lexus chap didnt believe the logs would be clear having felt the problem but checked them himself.
They've changed the coil and lead on 6 of the plugs which is not that hard to do I'm told. Two are difficult to get to and they havent checked those as it's a big job and if they do those they might as well change all 8 spark plugs again, or perhaps change four and then the other four.
They've done a smoke test on the inlet manifold and tested bits that Lexus changed under the recalls recently - the fuel rail I think they said - to be fair it was doing it before the service and recall work.
We've concluded that it doesnt seem it's about to break down and the dropping MPG is probably just me enjoying the late summer sunshine and doing more A roads and less motorways.
I'm going to drive it some more and we'll try again if it gets worse. Or if I can give them the car for a week with some notice they can change the plugs and coils, including the hard to get to ones.
SO MY QUESTION IS, for those who know the ISF better than I do, do we know of anyone else who has had this, maybe in the US?
Lastly, tech one havent charged me as they havent mended the car. Can't say fairer than that.