Re: failed the mot
Its always good practice to take the car for a reasonably energetic 20 mile drive before an emissions test to get the cat good and hot ands working. I've sold it now and sent the data sheets with it, but my 24valve V6 was always as clean as a whistle after a new lambda sensor in 2003.
I don't think pouring ANYTHING down the engine is a good plan at all. It could easily kill the catalyst. Cats last an amazingly long time as long as you don't poison them with lead or put raw fuel in them, but the O2 (lambda)sensor is a legitimate service item every four or five years I suspect (though our Skoda one has done nine years so far). Most decent garages can test the lambda signal and see if its OK using a test/multi-meter.