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In London theres a few folk saying its just due to it being very dry or hot the last few years and there is loads of movement in the clay beds. Worth getting someone who has good knowledge to take a look. Everyones favourite Rodger Bisby says don't fill in seasonal shift cracks, leave them open if poss (cover from weather if in an exposed location) so that when the ground wets up again house will go back to its former position.
Where is it, and how old? This summer was nuts for causing Victorian buildings to settle a bit more. Cracks galore.