Success!
Thanks for the advice.
The pin to the front was siezed solid.
The trick I needed, as posted above by Den, was to remove the rear pin from the mounting plate. I had to insert a second bolt to push the pin fully out as there is a slight recess in the mounting plate. (The compressed gaitor was pushing the pin back into the recess, so the bolt helped keep the pin out of the recess). Now I could start to rotate the two parts of the caliper. I initally had to use a small piece of wood to get a bit of leverage, to help get things moving. Once there was tiny bit of movement, it was just a case of rotating the two parts back and forth until they free up and the two parts seperated.
Once the seized pin was out I squirted some WD40 into the recess then placed a piece of plastic (from a long raw plug) in a drill and spun it round until the rust type layer disappeared. Cleaned out the recess so it looked sqeaky clean. Then smoothed off the offending pin with very fine W&D, lubricated with WD40. Polished it off and lubricated the pins lightly. Checked that the two parts of the caliper now moved completely smoothly in and out. Reassembled the caliper, gaiters etc. Washed up and then inserted the new pads. Dont want lubricants on the friction areas!
Hey presto the caliper just seated like a dream onto the disk! Torqued everyting up and jobs-a-good'n.
Ready for the Llangollen rideout which included a free brake test from a lamb no less.
I think as Den infers, I best lubricate the offending pin every time the rear wheel comes off, for new tyres etc, rather than wait until the pads wear out.
Thanks again to the GS knowledge Base
No wonder it took them so long to get to the moon 
