First off, 45 psi is within the Bentley spec. + or - .5 bar is roughly 7psi. Your problem is not likely fuel related imo.
Secondly, does your car have the filter with the fuel pressure regulator built in, or the external one? Edit: Guess the 330i has it built into the filter.
Fuel pressure can only leak via the check valve in the pump itself, the injectors, or the fuel pressure regulator on the return line afaik.