Citroen Xsara Picasso car struggling to start when left overnight

the car was fine until it turned very cold, since then it has struggled to start sounding like a flat battery, it had a new battery but it still did it next day, a new starter motor was fitted but still doing it, all the connections we're cleaned on cables to body but still no different, it starts perfectly when warm, it sounds like there is not enough power to pull the solenoid in when cold, any advice would be most welcome thankyou.