How to remove an airlock from underfloor heating tubes?

Our underfloor heating does not work in one room. The tubes in the floor of this room are fed from below, meaning that they are the high point on their circuit. There is an air lock or air block in them, preventing the water from circulating. Can someone suggest how to remove this airlock ?

View this post on my blog: http://madeinhome.info/how-to-remove-an-airlock-from-underfloor-heating-tubes-2