As we are all saying, this is not a job for the inexperienced and/or faint hearted. Without some considerable experience and all the correct kit, it's very likely to end in tears.
I was also, thinking laterally, that with only 20,000 miles on it and if the rest of the vehicle is in well above average condition, it could be worth spending the money involved in taking the head off. I say this because, although not a big problem, we all know the cylinder head gaskets on these can fail if either the engine has been very hot at some time or they can fail simply due to age deterioration - often blowing on No1 cylinder in my experience. If the rest of the vehicle warrants it you could be killing two birds with one stone by repairing the plug thread (and it will give you the opportunity to closely examine the other 3) and fitting a new head gasket on reassembly. The problem for Diamond will be if he can't do this for himself as labour cost will be considerable. However he says in his first post - at the top of the thread - that he's used helicoils before so if he does all the labour himself I think taking the head off, checking it very carefully, including the flatness of the face, and doing the repair on the bench if it all checks out would be the way to go. Good opportunity to put a new timing belt on during the rebuild too if it needs one.