NavList:

Geoffrey and Bruce

Thank you for that really useful information.  Practical results are always good to hear and stow away for future use.

Frank you said:

 But are you sure you really need that? How would you use that point?

You’re correct of course.  I realised that myself having slept on the matter.  What I really need is the relative positions of three points about 50 metres apart quite accurately and I can do that using a 100m tape measure and a compass or hand-held GNSS receiver to get a base line.  The matching of my three points to lat & long is desirable for what I want to do, but extreme accuracy isn’t necessary so long as all three points are wrong by the same amount.  DaveP