This info is from my boyfriend who was an insurance broker until the beginning of the year:
It will be possible to find cover but it would be hard, because the fact that you have the busines use, it apears to most insurance companies that you use the car quite a lot and are therefore NOT a second driver, whether this is the case or not.
Companies that might cover you that way are: Viva & possibly direct line and almost definatley Zenith.
Make sure you are taking out the correct business use, you need business use - class 3 as you are collecting money from the clients (also covers sign writing should your car have any).