Have just had to fit new cold water tank in loft, this is a supply tank to the old hotwater tank in the back bedroom. At first there was no water to the hot taps got that going so turned on immersion, the problem is when it got hot the ventpipe/overflow keeps running where have I gone wrong?

By "vent pipe" I assume you mean the safety valve that opens up if either the temperature or the pressure exceeds the valves limits. We can rule out pressure since the hot water tank's being gravity fed from the lot. That leaves the temperature. S the water boiling hot? If not I'd suspect the valve itself. Replace the valve. Good luck, Tom