Adjusted black marker to be level with red marker on 1 when boiler turned on for heating black marker moved to nearly 2 boiler turned off and blacker marker fallen back to 1.5. Should both black and red markers be at 1 when turned off?
Red needle should be set to about a 0.5 so you know when to top up system.Black needle should sit between 1 and 1.5 when cold.When hot it may rise to as much as 2.5.This is ok.If it goes near 3 then you have a problem and will need a Corgi Engineer.