l have noticed that on some rare occasions my bmw, struggles starting up, when l press the ''start'' button. It starts but not immediately as it should do.
Why can this be? The car is new,a few months old. l drive mainly in town. Could it be the battery and the fact that the car is driven no more than 45-50miles per hour? l cannot determine what is this due to. The car is driven almost every day.

It is brand new, take it in and tell them exactly what you told us. They will look into it for free. It is under warranty. Ca not tell you if it is the battery or not. You said it eventually starts, but does that mean it cranks over slow, or just that it cranks and cranks and then finally fires?

While it is possible that your battery is low in charge, there are 2 currently common concerns about a 335.
First is the high pressure fuel pump.
2nd is the ignition switch system in the steering wheel (CAS/ELV).
Both of the above can cause your problem. Bring it to a dealer for proper diagnosis and repair under warranty.