Hi
I've found the answer in using Great Circle calculations and the co-ordinates for tthe ends of the runways included in the data file.
Even on short runways using a simple pythagorus triangle solution add in too many errors.
The formula in Excel is
=ACOS(COS(RADIANS(90-A2)) *COS(RADIANS(90-A3)) +SIN(RADIANS(90-A2)) *SIN(RADIANS(90-A3)) *COS(RADIANS(B2-B3))) *6371
Co-ordinates in A2,B2 and A3,B3
Thanks to BLueMM for the answer. Multiply this result by 1000 to convert to metres in length.
This formula gives a 2m error on the published length of Heathrows 3904m runway using the data in the current airport data file.
Bob