The trailing arm is the big lump with the hub attached
good picture is here:
http://www.amazon.co.uk/Ford-Focus-Rear-Suspension-Trailing/dp/B0035923NU
It would not be usual to change that whole lump (4a493) I imagine it would
cost a small fortune the bush that attaches to the body work (5A638) would
be the only part you could describe as a trailing arm bush.
From the above image you can see that the more commonly replaced bit is 5500
A
None of the above parts is that hard to change from what I saw underneath
today, as to actually changing that bush in the front of the arm .......?
Most garages should have a press, or there may even be a Sealey tool to
change them in situ, but it would be out of the scope of most home
mechanics.
I would still leave it alone until it starts clonking or fails the mot, even
massive failure of the rubber bush would not make a huge difference to the
handling or safety since the bolt through the middle would restrain
everything.