Yup, there should be a small "R" clip which needs removing, then knock the pin back with a narrow punch. It needs to be narrow to fit through the hole in the back of the calliper.
Before removing old pads, put your knee against calliper and give it a good shove to push the fluid back to the resevoir.
Now, I've done this a couple of times in the past with no problems.
But this time I cannot get two new full size pads in when the disk is in position.
The cylinder is pushed all the way back.
The pads sit fine when the wheel is out.
The wheel spins normally when the pads are out.
It seems the overall brake caliper is too close to the swinging arm, meaning there is not enough space for the 'inner' pad (the one nearest the wheel) to fit.
I'm sure I have the wheel spacers the right way round, the bigger one is on the side of the brake.