I read with interest Elemen of Feb 2010 as my boiler has recently started to do this. I suspect it could be the main control board (clicking away) but that is expensive to replace if it the Temperature Control, or the Sensor, that causes this? Did the problem get resolved?
I also read about Rays problem last year. I had boiling water being dumped in the header tank and although I initially discounted the cylinder stat, it did turn out to be 'sticky' and I replaced it for £9 (Ebay seller)

Terry, December 2010