Get center f geocodes

I have the 4 corners of a map, in geocodes. This seems OK, but im not
sure if I add the 2 x longitudes together and divide by 2 and then do
that with Latitudes too, will that give me the center values?

However, I've decided that won't work. if we did that with a town East
of GMT and West of GMT, the center will be in the southern hemisphere
somewhere - not in the UK.

If I get the 4 corners, is there a small script to get the center point?

-- 

Gordon.