Thanks for the advice folks, job done. A final observation and question, I presume the higher drain tap allows an area to be drained which would otherwise not be? I did take this tap out and replaced it with a connector and hose, by loosening the pressure cap and allowing the water to reach the overflow pipe and then turning off and allowing to drain did shift a little more debris. Used Holts 2 pack rad clean, worked to a point, teaspoon of debris rather than the shovel load I expected after seeing the sludge when the core plugs were removed last year.
John