Thanks to everyone for ideas, warnings and experiences. Lots of good sensible and interesting advice given.
I think that I might have underestimated the strength of the roots and overestimated the power of the car before making the original post!
Coupled with the dangers of ropes snapping and the potential energy contained by them being released suddenly with a danger to life and property (sorry to hear about the window Dave :-) ) and the possibility of damage to drains etc., I have decided that they will just be reduced to stumps and left for a couple of years to weaken the roots before any extraction is undertaken. Thanks Phil. L for the suggestion.
I presume that these stumps will not need chemical treatment to kill them off as I believe that conifer stumps will just die anyway. I wonder whether chemical killing would hasten the weakening of the roots?
Once again, this group has come to my rescue and brought me down to earth!
Cheers
Steve