Recipe as provided by FatherGoose, expert in many things Citroenical.
Step 1/ Go to Autoparts store
Step 2/ Buy bottle of engine oil flush
Step 3/ Open LHM reservoir in engine bay
(I think you are probably getting the drift no)
Step 4/ Open engine oil flush bottle and pour entire contents into LHM reservoir
Step 5/ Close LHM reservoir and bonnet
Step 6/ Drive car for at least 1000km (not continuously of course) and put the hydraulics through a few gymastic lessons (ie full up and down cycles)
After all this, proceed to normal routine for a full change of LHM, including bleeding brake lines.
Did this to the XM and the gunk that came out was incredible.