My colleague got an AC adaptor for his Amilo D1845 from an eBay shop by searching for the manufacturer's part number. It is a 20V 8A adaptor and it cost around £80; expensive, but still a lot cheaper than Fujitsu-Siemens.
The original PSU had a flashing (blinking) green power light and delivered no power. The fault turned out to be that the cable had frayed internally between the plug that goes into the laptop and the lump that contains the anti-RF ferrite choke. The cable was short-circuiting there. Half an hour's work with a modelling knife, soldering iron and
self-amalgamating tape and he has a working PSU.