Thanks George.

I suppose what's most confusing me most is not the bearing, but how to come at this 'error' figure.

To go back to the very first example, that is Maualtanvally circle to Knocks S circle to Knockatlowig row.

(N51.64614 W-9.06488) to (N51.64749 W-9.00848) to (N51.64805 W-8.98150)

I initially got a bearing between Maulatanvally and Knockatlowig of 87.853 and a distance of 5.749km, then a bearing between Maualtanvally and Knocks S of 87.769 and a distance of 3.899km.

I then treated the two lines as the two longer sides of a right-angled triangle and applied:

sin(difference between the two bearings) x length of side being measured to = perpindicular distance to that side

to (a) get the distance error between Knocks S and the Maulatanvally/Knockatlowig line (3.899km); 5.72m error
(b) get the distance error between Knockatlowig and the Maulatanvally/Knocks S line projected eastwards (5.749km); 8.47m error

I wasn't sure if this was the right way to go about it, or even if my maths were correct, so when I saw that you got 26m error for the same coordinates I assumed that I was incorrect.

So, I guess my question is, what is the correct formula for sorting this out?

Thanks again.