The present invention relates to a cryostat having a service neck for access to a superconducting magnet. In many cryogenic applications components, e.g. superconducting coils for magnetic resonance imaging (MRI), superconducting transformers, generators, electronics, are cooled by keeping them in contact with a volume of liquefied, the whole cryogenic assembly being known as a cryostat. In order to operate a superconducting magnet, it must be kept at a temperature below its superconducting transition temperature. A cryostat must provide access to the vessel containing the liquefied helium for the initial cooling of the magnet to its low operating temperature, for periodic refilling of systems where there is a loss of helium, and provide sufficient access whereby to enable operation and maintenance of the magnet. The present invention seeks to provide an access neck to a cryostat such as helium vessel with a minimum heat load and accordingly provides a cryostat assembly, wherein a service neck comprising at least one positive and one negative current lead is arranged such that one of the leads is formed by the neck tube wall and the space between a neck tube wall and the second current lead forms a gas path for venting and/or filling or other services.