A 10/4 Cistern, (Tanks are sealed vessels such as Hot Water cylinders), of the type/size usually used for Feed and Expansion on central heating systems, are 10 gallon actual capacity. So with 4.542 litres to a gallon, the maximum capacity can only be 45 litres.
As long as there is enough water in the cistern to cover the outlet at all times, it'll be fine. If the ballvalve is knackered, replace with a Part 2 type, and set the float as low as possible to begin with, see how that works.
As long as there is enough water in the cistern to cover the outlet at all times, it'll be fine. If the ballvalve is knackered, replace with a Part 2 type, and set the float as low as possible to begin with, see how that works.