Big Thanks to Smiffys for helping me with a little bit of advice on this one.
Basically, I did the rears and could see that the guide pins moved freely in those, so I could see exactly how the floating mechanism was supposed to work. I phoned Smiffys to get some advice on what to do over the fronts, which had two guide pins siezed on the nearside and the upper one siezed on the offside. I was told that the pins obviously needed free movement and that getting a bit rough with it was the only way that I was likely to be able to free them. (It was a great help getting advice from someone who had been through the same situation.)
Armed with the confident advice that I was not likely to do much damage if I got fairly brutal with the siezed offside one. I turned the guide pin with a 17mm socket on the end nut, added a liberal spray of WD40, and a friend tapping the casting to help free it. It only took a few minutes to free, then clean it up and regrease it and, Hey Presto!, a working floating caliper mechanism. Armed with my new found confidence I managed to free the nearside ones much quicker. Haven't tested it yet as it got dark and I was up to the eyeballs in muck, but hopefully I now have a soarer that stops.
It also made me think that with 3 out of 4 guide pins siezed on the front brakes, my braking efficiency must have been seriously reduced, which I thought was due to needing new pads. I suspect I will notice the difference now though.
Thanks guys.