Help! How can I make the hot water flow faster?

We have never had any trouble with our hot water flow until we had a new Glow Worm condensing boiler installed about 18 months ago. I noted that the flow of water from the bath tap was not as fast as it had been almost immediately. The plumber advised me that it was fine but some weeks later the flow stopped altogether. The plumber said that there was an air lock and showed me how to fix it by taking the mixer tap off in the kitchen and forcing the cold water back up the hot water tap. The trouble is that the problem keeps recurring. We are continually having to rectify it ourselves. We have had 2 plumbers out and they vaguely talk about draining the system and then don't come back. If we run the hot water to the bath, we can't get any water out of the sink tap at the same time. It's driving me mad! Could it be that there is some sort of blockage. I've been up to the tank in the loft and there is some sediment in the bottom which I think has come from the cover which lagged the tank.