Like you I think I'd be tempted to give it a go, but any swarf in the engine could spell disaster for the valves so garages wouldn't be keen to do this. The plug may come out if you can find some way of pulling upwards as you unscrew it - if it does come out there may be enough thread to pop a new one in. You could price up an exchange head from a breakers I guess, or ring up a few engineering shops to see what they say - they will have seen this before.
I haven't taken off a diesel Mondeo head yet but I have done petrols - they aren't so bad but you need to familiarise yourself with the camshaft timing marks before you get stuck in. Good luck with it!
John
March 2008