The cooling system is partially pressurised (so boiling point is higher), and is controlled by the expansion tank pressure cap.
The tank between the cylinders is a "catch tank" for any coolant expelled as the system heats up and expands. Any fluid that is expelled will be "sucked" back into the main cooling circuit as the engine cools.
Most bikes that come into the workshop have a bit of coolant in the middle tank.......
A new cap for the front expansion tank is £8.16, and they can cause cooling problems.
Good luck at the BMF, make sure you remember where you park the bike
[Edited on 17-5-2006 by mitchc]