i'll try wrap all possibilities for you.
The expansion vessel for the ch system on the puma is the large round drum located on the very back of the boiler behind the combustion area. you should be able to get to a valve if you look down over the top of the boiler.
if you remove the cap and press the pin in you'll get an indication of your problem.
1: if you get air- possible blocked pipe to expansion vessel not allowing system to expand in vessel.
2: nothing- possibly flat and need pumped up.
3: water- possibly dead.
Never replaced one on this boiler so cant give you hands on advice but looking at location dismantle flue elbow drain boiler and drop new vessel in and connect pipe up to vessel.
If the engineer thinks it could turn out to be a nightmare then a remote vessel could be fitted if it would not cause a problem to locate it. remote vessels are not the same as boiler ones but can be used if an akward situation.
the small round vessel you refferred to was domestic hot water vessel.
hope this helps.
iain
May 2008