Well I have managed to stop it intermittently alarming on overheated return from the heating circuit (though it still cycles OK in this state the output is reduced and the house cools). This by reducing the speed setting on the house circulation pump (max to min of 3) and putting on more radiators (top floor /3 was off whilst we painted). The house radiators are still all hot. Also by backing off the 'slope' to the factory 4.0.
Perhaps counter-intuitively this actually raises the house temperatures (to normal, meaning 19+-0.3 for rooms) and raises the GSHP output (drops the cool ground circuit temperature), partly of course by cooling the return heating loop - but the controls are strange all the same. I see that it is possible to have a temperature-dependent slope; this may be the answer, 4 when it is cold, more when it is warmer.
FTR external 2m above ground temperature is -3 to -3.5C, the same as last night.
Note added 28/11/10:-
This seems to have worked well; mainly because the GSHP cycle time is for unclear reasons shortened (by about 20% for a slope change from 4.5 to 4) though the change in house pump circulating speed may play a role. External temperature last night fell to -4.4C but we are still some distance clear from the return 'limit'.
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