If your business is registered at your home address then you will claim from there and back and any mileage you do which is business related in between (including going to the suppliers etc)
The easiest way to do it (if you are organised) is to keep a notebook or something in the car. At the start of any business journey, take a note of your mileage, then do the same at the end. If you are in your car going ANYWHERE that is business related, claim your mileage! Remember and keep your petrol receipts.
If you are in any doubt (and can bear it) there is a section on the HMRC website that explains in more detail.
Elaine