Heights above sea-level seem to be a confusing area. WGS84 gives the best fit to the earth world-wide but there are significant local difference. According to the UK Ordnance Survey(OS), WGS84 is about 50m different from the Airey Ellipsoid which gives a better fit for the UK. Also, there's 10m difference between the east and west of the UK.Incidebntally, the OS give a method of calculation to go directly from lat, long, and height to x, y, and z but only gives an interative calculation to go back again. Do you know of a direct method?

I converted the calculations in Appedix B of this .pdf to my C code.http://microboticsinc.com/midg_files/M2DCPUG.pdf