The Halstead part is based around the diverter valve section. The diverter valve is a directional water device that only appears in combi boilers. It allows the hot water to be sent to the heating system or to the domestic hot water system. This feature is what makes a combi boiler so popular due to the fact that a combi boiler wouldn't require a cylinder to provide heating and hot water.
Fits a number of Halstead boilers
Technical specification
Waist Sizeab22cf14fwTypeDiverter Valve Service KitColourMixedSupplier Part Number840503Brand NameHalstead
Reviews
Halstead Diverter Valve Inner Service Kit Mixed 840503