Worcester Bosh Boiler 24/28 si II?

I have a Worcester Bosh boiler that keeps losing pressure from what I believe to be the pressure release valve (the pipe outside the house). I have had a heating engineer out, who has fitted a new expansion vessel. I am still losing pressure but only a small amount but I am now having further problems...... when the heating is fired up and has been left on for a while, when I turn the hot tap on i now don't get any hot water and the boiler indicates a fault with a flashing light. When the boiler has cooled off and I reset I have hot water on demand. What is the problem.... I hope someone can help!!!! Many thanks