I've recently changed all four bushes on the front arms of one of my Xantiae;
two each arm as you say; the mushroom one at the front and the larger one
for the rear. These are BOTH available from GSF.
In hindsight I wish I'd left the mushroom one's alone as they weren't
degraded/worn and were perfectly OK on the arms. These were an absolute
swine to remove and put back into the arm. Took me a whole afternoon
faffing about and unless you can burn these out with plasma/oxy I'd leave
well alone unless of course you've got nothing else to do or they have for
some reason really gone pear shaped instead of mushroom shaped!!
The rear bush is an all in one piece part which you replace the whole lot
with the new 13quid one from GSF - they are handed and you MUST get
their orientation right - not only in terms of the physical direction the bolt
through bracket faces but also the angle of their location on the arm as
the arm needs to sit off the subframe dead horizontal before being
attached to the strut by the balljoint - it's all shown in the Haynes.
These can be hacksawed or carefully angle grindered off but don't be
tempted to clean the wishbone arm's shank if you do slightly catch it with
the grinder and take too much off the diameter

- a tad over zealous
with a file I had to get a different arm - doh!!
I did find the listing for GSF seemed to contradict the physical packet's
labelling of the rear bracket/bush and the guys mere adamant that the left
hand one they wanted to sell me was right until I showed it alongside my
car that was parked outside!! Possible a mislabelling in their stock, but
something to keep an eye out on to save potentially wasted journey's to
and fro!!
Andrew