Thanks for the reply stem. It's all sorted now thanks. It was an airlock in the system. I bled my invented cylinder and to make sure I also opened up all the taps. Boiler now fires up and thankfully heats the water up. At least I know what to do when I flush out the cleaner next weekend, fingers...
My Honeywell smartfit system has stopped heating the water up. Over the weekend I drained my CH system with a view to flushing it out. I isolated the boiler, (turned off the power) and drained my CH system. I added the cleaner to the expansion tank and refilled the system. My CH works perfectly...