For the pressure to rise overnight either air or water is being added to the system somehow. Is your filling loop a permanent connection? If it is I would suspect that the valve in the loop is 'leaking' mains pressure into your heating system. You could perhaps verify this by turning your stopcock completely off at bed time (ie eliminating mains pressure from the equation). If in the morning your system pressure is correct then you have a faulty valve. I can't think what else it could be!
Richard
December 2008
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