Back after an interesting few days away...
Got the answer I needed. Quickly realised the need for accuracy in decimal angles at these latitudes. It gave me an answer to within 3 miles using only one decimal point, which for my purposes is close enough.
Used the program you gave - a good little unit, but kept getting wildly different co-ords. Finger trouble I think! Switched to Ed Williams avformulary excel spreadsheet and worked a treat.
Unfortunately, given the shape of my long skinny triangle at this lat (between 66* and 81*), the point seems to be outside the triangle
Rechecking and redoing sums now.
Cheers
CS