What is causing my overflow pipe to drip now ive turned on the heating?

I have recently turned on the heating in my 3 bed house. Since doing so the smaller tank in the loft(the other is twice the size) is filling up and water is dripping out of the overflow pipe by the roof line at the back of the house. I thought it was caused by the radiators needed bleeding, this done it stopped....but it might have been a fluke as 3 days later it started again.....and is still dripping???????

J. SHAW
October 2004