I also previously owned a "p" mondeo td and experienced a similar problem. I'm no mechanic but what I remember about the problem was that the uncontrollable revs were caused by fuel and oil mixing. This was caused by deterioration of a rubber non-return valve in the turbo(about 50p) Unfortunately I had to replace the Turbo with a second hand unit. I think the garage charged me about £50
pounda, October 2008