Brilliant, thanks wetfeet!
I think this conclusively shows that it's actually very straightforward to draw detailed contour plots (and that 1970s style arguments about computing power are a bit old hat).
Trust the Norwegians to quietly and competently do it better and more precisely. They say: "Dette er tilleggsinformasjon til VAAC-bulletinene, som utstedes av Volcanic Ash Advisory. SNAP-modellen gir en beregning lenger fram i tid, fram til 66 timer, samt større oppløsning i tid og rom" <translating> "This is additional information to the VAAC bulletins, which are published by Volcanic Ash Advisory. The SNAP model calculates further forward in time, up to 66 hours, and greater resolution in time and space".
So, the Norwegians have a different model from the UK - I hope someone's talking to them to draw on their experience and not doing the traditional "not invented here". In fact, wouldn't it be wonderful if someone (from, oooh, let's say the regulators) called all the chemists, meteorologists, atmospheric physicists, jet engine engineers, flight ops professionals, etc. together in a multi-disciplinary team to sort this out once and for all. And, it's no good saying "that's TFD" - if you can't finish don't start, with "zero tolerance", which was never actually "zero" but a 0.2 mg limit because that was chosen as the model cutoff (quite arbitrarily, as far as I can tell)...
For the scientifically inclined, here's plenty to chew on:
http://www.skybrary.aero/bookshelf/books/1162.pdf
including a paper: "IMPACT OF VOLCANIC ASH FROM 15 DECEMBER 1989 REDOUBT VOLCANO ERUPTION ON GE CF6-80C2 TURBOFAN ENGINES". This states that the KLM Redoubt encounter was at a range of 150 nm and that the estimated ash concentration was 2 grams per m3 i.e. 500 times denser than what is currently being claimed to be dangerous.