Stuart wrote:

That "bit of metal" as you describe it is a sash weight ..usually cast iron with
a loop at the top to take the sash cord. I'm trying to figure out why the
window won't stay up as it won't have got heavier and the sash weight won't have
got lighter ....It might be an idea to have a look in the sash box ( by removing
a panel in the runner ) and check the condidtion of the rope anyway ( remember
to support the window if you are removing the weight or lifting it up )
otherwise you might get a sore head or worse . It could be that one of the
weights ( there is one per side ) has become detached and causing your problem

This is probably the case. Remeber that some sash windows don't
actually have rope and weights but I guess this might be clear.