Thanks to the support guys from the Apple shop called 'The Innovators' (Tilburg, Netherlands) the problem was quickly fixed as follows:
- If you don't have one, then buy a FireWire 400 connecting cable, so with the same connector on both ends.
- Connect 2 Macs using the cable.
- Restart the faulty one and hold T while it boots. You should then get the FireWire logo on the screen. The harddisk of the faulty machine is now mounted on the other one.
- Download the combined Mac OS X update.
- Run the update package from the good machine.
- When asked for a destination, select the other machine, connected via FireWire.