the pressure is set cold at between one and one and a half,
when heating is on pressure rises slightly as it should, however when switched off overnight the pressure rises to about two and a half, bleeding the radiator reduces the pressure again.

A Griff, December 2008

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