Installation manual: http://www.ferroli.co.uk/wp-content/uploads/downloads/2012/10/Optima-901-Manual.pdf
Page 10 and 11 for instructions and pictures about "Make up Water". It sounds as if the system is permanenetly slowly fiilling up. Locate the filling loop (Picture of a typical item on P11) and ensure it is turned off. If in doubt consult a plumber.
Allow to settle and reduce the pressure to design figure by bleeding radiators.
If pressure continues to fall with the filling loop fully isolated, you may have a leak either in the boiler, its pressure relief valve or radiator or pipework.
Work carefully and methoidically. If in doubt consult a plumber.
Do not attempt anything on the boiler iuless you are Gas Safe certiifed.
users instructions: http://www.ferroli.co.uk/wp-content/uploads/downloads/2012/10/Optima-901-User-Instructions.pdf