Apologies for the delay.
The Groundspeed trig goes as follows:
GS=(D20*SQRT(1-(SIN((H20-G20)*PI()/180)*J20/D20)^2))-J20*COS((H20-G20)*PI()/180)
where:
D20 is the TAS
H20 is the Wind Direction
G20 is the Desired Track
J20 is the Wind Speed
The basic calculation was lifted from
http://www.best.com/~williams/avform.htm#Wind
so respects to Ed Williams for understanding the trig.
Actually Ed's calculations include a subtle test to ensure that the course can actually be flown. e.g. if the w/v is 100Knots from 270degrees, and your TAS is 90Knots you cannot fly a course of 270 - cos you will be going backwards
I omitted this test from my spreadsheet on the basis that I would never plan to go flying in a wind that was capable of pushing me backwards!
Enjoy