Peter, thanks again. Once I ignored the workshop manual the job became fairly easy, as long as you remember to undo the 'air cleaner retaining bolt'.
So, I did the job, all as described, and it was certainly not a waste of money. The test run revealed that the engine seems to rev much more freely, but unfortunately the amber light and restricted performance still came on and is doing so as soon as I give the accelerator a little umph. But as I said earlier, stop, switch off, restart, all back to normal.
So, air flow cleaned, now on to the fuel. I will use one of the additives (I read that Archoil AP6400-D may be as good as any?) and see if that helps with the Amber light problem.
Keith