Is there a single lockshield valve on old CH system?
 

My parents have an old single pipe CH system and the last radiator on
the end of the system only gets hot at the top and is cool at the
bottom. I believe this is a balancing problem and if I turn off other
rads this one will get hot. Now the problem is that this is an old
system and the radiators (apart from the last 2 which were added later
with the extension) have no lockshield valves fitted, only the
open/close valve on one end only

Ideally they should have the system upgraded with LSV's on every rad but
I did read a similar post on google and someone suggested that there may
be a single lockshield valve fitted for the whole system. If that is
correct could someone please tell me what it would look like and where
this might be located. They live in a bungalow so would it be (if
fitted) somewhere in the loft?
Thanks.