How can I replace the Rear Disk Brakes on a Vauxhall Vectra 2.0 DTI?

Vauxhall Vectra 2.0 DTI on a "Y" plate (last of the model before the 2.2 series).
The rear brake linings are worn to limits, but I can't see how to remove the 2 pins that appear to locate and secure the pads in place.
There is quite a lot of dirt and corrosuion build up on the caliper, but after a little cleaning, there still appears to be no visible means of extracting these 2 pins!
Can you help?