how can i fix repeated low pressure of my puma 80 boiler?

I noticed a few weeks ago when i woke up that the boiler was shining 'low system pressure' so i topped it up and the heating worked fine. a week later heating and hot water was stopping again with the same problem. i can easily top it up but would like to know how to fix the problem because there is obviously a leak but i cant find one in the house?