Morning Tony, hope you are both settling in well.
I had this problem a few years back with a neighbour complaining about a row of conifers blocking the light to his house, so i set about reducing the height from 15ft down to 6ft, i had one hell of a job trying to reach the trunk 6ft up so that i could cut though, i think you can picture it trying to lean a ladder against a conifer, so have you worked out how you are going to do this 15-20ft up and i should think that the diameter must be about 6-8ft that high up, how are you going to reach the trunk?
And by the way 75% of the conifers that i reduced died on me and the rest looked terrible.