Thank GJ - usefull stuff.
Didn't get this though:
Originally Posted by Goldfish Jack
... as controlled airspace has to be constructed to ensure terrain does not appear (for want of better words) in it. (That is why the lower limit of controlled airspace is always a VFR level and is always 700ft above the highest obstacle within 5nm. Hence if you are IFR and fly at the lowest IFR level of controlled airspace you will always be at 1200ft above the highest terrain within 5nm of your position.
ATZs and CTRs surely qualify as controlled airspace?