Assuming all 155's are the same as the 16v in this dept, Pull the black plastic cover off the relays on the battery tray, pull the relays off one at a time using a flat bladed screw driver to release them, and voila, enough space to get at the back of the headlight to change the bulb. PM me if you run into trouble and I'll guide you though it over the phone if you want.
ive allways managed to do them without removing anything apart from the rear of the headlight plastict just unplug the connector for the light and then the actual connector unclips from the plastic(white) rear cover. then release both of the clips holding the cover on and thats it.
on my 8 valve there are no relays behind light bnt the space is blocked by a plastic and rubber hose from the management system i think and i did not really want to have to mess about taking it off but i think imay have to.