I'm well aware that volcanic ash and gas turbine engines are not good bed fellows: I saw first hand the stripped down RB 211s that had been involved in the British Airways 747 incident in 1982 and, represented Rolls-Royce at meetings with BA where we argued about the precise failure mechanisms! BA said that the ash built up to such an extent that the internal nozzles blocked and caused compressor surge and RR maintained that the engines flamed out due to reduced O2 (or was it the other way round?). I suspect that both mechanisms came into play but the engines started again and everyone walked away unscathed!
On a lighter note, the Cherwell Valley Services have burnt down within days of Mrs Nikos suffering the worst manifestation of English greasy over priced breakfasts there. Whoever set fire to this establishment has done the public a good turn!
============================================
UPDATEThis is the sort of cobblers that's gets my goat on the BBC web site (copied, I suspect, from the equally factual and objective Daily Mirror) - completely stupid and misinformed reporting missing the most important factor!
Yes the consumption of volcanic ash might ultimately erode metal and the other things mentioned but the main problem is it blocks the compressor and air flow and causes flame out or compressor surging = engine stops!