Hello, Paul,  I understand that you are looking for a brass pressure
bar for a barrel piano.  I'll bet you will have a tough time trying
to buy one new or even adapting a bar from an old piano.

Many times I am asked where I got the parts to restore some
instrument that was made 150 years ago.  I usually smile and point
down to my shop in the basement.  Other than piano parts and a few
common parts for music boxes, you have to make them.  Barrel organs
and pianos?  Forget it!

But, from your description, I think you could make a pressure bar
yourself without much trouble.  In the USA, we have a great company
called McMaster Carr.  They ship materials the same day at very
reasonable cost and minimal postage.  The URL for their copper/brass
materials section is   http://www.mcmaster.com/#copper/=crsw3k 
I'm sure you can find a similar source in the UK.  In fact, many
scrap metal dealers come across the same material at much lower cost.

Make a "V" groove along the side of a straight piece of hardwood to
support the round shape of the brass bar.  Make a flat on each end
of the bar (and maybe another in the middle) so you can screw the bar
into the groove so it won't turn while you handle it.

I have a tapered sanding disk for my table saw so I would pass the
brass bar/hardwood mount between the fence and the sanding disk
several times to make a flat on one side.  Drill and countersink
a few holes and you have a new pressure bar.  You could also do the
same thing with a drum sander and a fence on your drill press or
spindle sander.

I'll bet it would take less time to make a pressure bar than you've
spent calling around to find one.  Good luck.

Craig Smith
Upstate New York