Small tank in loft is leaking water through the overflow pipe?

The small tank in my loft is the one which is leaking water from the overflow pipe onto my decking area at the back of my house. It varies and can overflow a lot at a time or a little bit at a time. How can i fix this i have removed some water and it stops leaking water from the overflow for a day then returns leaking a little the next day. How can I fix this and does one have to turn the water off and if so where would i find this.

Thanks