Had a re-think last night and came to the conclusion that the best way to get the outer bushes into the spring pan arm housing nice and tight would be to insert the two halves by hand without the metal centre, this way they are in seated in position and you know they wont try and pop out as there is nothing working against you like excessive air/grease etc that gets trapped/or builds up and pushes outwards when trying to insert using other methods.
The next trick is to get the metal centre into the bushes without dislodging them.
So partially insert the metal centre into one half of the bush (remember these are already in the housing) just a few mm will do, then thread a long bolt through the large outer washer (used on the arm when installed and the correct diameter, the same as the bush). Insert this through the bush on opposite side of the housing and then through the bush with the centre inserted, put a washer over the open end of the bolt and onto the metal center (about the same diameter), fasten the bolt above it. Get tightening this up and you can draw the centre through the two halves of the bush whist keeping sufficient pressure on the bushes so they are kept in place.
This seems to work for me, fingers crossed, I have left both arms bolted up just in case and will undo tomorrow.
Thanks for your help.