If the vessel is fine, jump in the car and take a trip down your local plumb merchants; buy yourself a tube (Upto 10 rads) of "Central Heating Leak Sealer"... Might solve your problem without tracing pipes.
As it's a sealed system, save yourself some time and get yourself a rad injection kit with it as well... Your plumbing merchant will be able to give you some advice on how to inject the liquid into your system.
GasAid.co.uk
December 2006