All radiators get warm but they don't sustain the heat and go off after a short while .The circulating pump is new this is a pumped system.

You may have a problem called air lock. There is air in the radiators causing problems to the circulating hot water. There should be a bleed valve at each radiator. On the web site How do I Bleed a Radiator? (http://www.wisegeek.com/how-do-i-bleed-a-radiator.htm)
They give the following instructions: To encourage your heating system to work at optimum efficiency, it is important to bleed your radiators on a regular basis. Though it may sound difficult to bleed a radiator, it is actually quite easy to do. There is no special training required. All you need is a radiator key and a small bowl.

If the heat in your home isn’t circulating well or the bottom portion of your radiator is warm, while the top portion is cold, there may be air trapped inside your radiator. When air gets trapped inside a radiator, it reduces the radiator’s ability to heat effectively. This trapped air may even lead other parts of your heating system to function poorly. Sometimes, most of a radiator will fill with air. In such a case, there will be no noticeable temperature difference. The whole radiator will be cool to the touch. As you prepare to bleed a radiator, you will most likely be starting with a system that has been turned on. As such, there should be warm water in the radiator. Before you begin to bleed a radiator, you will need to turn the central heating system off.

To bleed a radiator, you’ll need to have a bleed key handy. With this in hand, locate the bleed valve. It is typically positioned at the top of a radiator, near the end. Insert the bleed key into the bleed valve and turn it counterclockwise. Don’t turn it too much; a half a turn is usually enough to successfully bleed a radiator.
As you turn the radiator key, the valve will open and you will hear a hissing sound. This is completely normal and is simply caused by the air escaping. Once water begins to leak out of the radiator, it is time to close the valve. To do so, turn the bleed key clockwise for half a turn. You can then move on to bleed a radiator in another part of your home, as necessary, before turning your central heating system back on.
Hopefully this may be your problem and bleeding the air from the system should be the remedy.