7 June 2010 : Since I wrote wrote on the 14 April that the GSHP seems to think it's a 'kettle', I have now got used to the overheating of the water in the mornings.
The GSHP is designed to be left on all the time, 24/7, and the time for 'Pasteurisation' can be set to a time difference, such as every 2 weeks, or every three weeks. So left to itself, the DHW will remain at or near the target, and perform economically with small heating cycles, and only have one bigger heat up every 2 weeks or so. Pasteurisation means boosting the water to 61º or 62º C. This is a safety precaution, in case Legionnaires bacteria have had time to grow in the tank.
What seems clear, now that we are well into the summer, is that the GSHP checks its internal clock at start up. There must be a measured time of 'Off' after which it will embark on a full Pasteurisation cycle. We don't know if it is 8 hours, 12 hours or what. It is somewhere between. If it's off for 3-4 hours it does not do this, but in the mornings after a night of Off (e.g. 10 to 12 hours), it does do this.
We have got used to it now, it does turn off correctly after reaching 62ºC, a full 11 degs C greater than the target temperature of 51ºC. The earth is so warm after a night's recovering and distributing the heat that was put down the previous day that the morning warmup cycle is acceptably quick. I would willingly leave it on all night, if it wasn't for the fact that it circulates water round the underfloor heating circuit at night, using pump power of 125W and making a small amount of noise. We don't want to be hot at night.