A refrigeration system comprising: a main refrigeration circuit for holding refrigerant fluid, the main refrigeration circuit including: a compression device, a heat rejecting heat exchanger, an expansion device and a heat absorbing heat exchanger; wherein: the refrigeration system includes a buffer tank attached to the main refrigeration circuit, with valves (22, 24) for controlling flow of refrigerant fluid between the main refrigeration circuit and the buffer tank; the refrigeration system is arranged such that the valves (22, 24) are controllable to transfer refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of sub-cooling in the main refrigeration circuit; the valves (22, 24) comprise: a first valve for controlling flow to or from a higher pressure point on the refrigerant circuit, and a second valve for controlling flow to or from a lower pressure point on the refrigerant circuit; and characterised in that: the second valve is a valve with a controllable degree of opening and the refrigeration system is arranged such that the degree of opening is variable whilst the refrigerant fluid is being transferred from the buffer tank to the main refrigeration circuit; and in that the refrigeration system is arranged such that, during operation of the refrigeration system, control of the degree of opening of the second valve is used in order to control the amount of liquid refrigerant fluid at an inlet of the compression device. a main refrigeration circuit for holding refrigerant fluid, the main refrigeration circuit including: a compression device, a heat rejecting heat exchanger, an expansion device and a heat absorbing heat exchanger; wherein: the refrigeration system includes a buffer tank attached to the main refrigeration circuit, with valves (22, 24) for controlling flow of refrigerant fluid between the main refrigeration circuit and the buffer tank; the refrigeration system is arranged such that the valves (22, 24) are controllable to transfer refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of sub-cooling in the main refrigeration circuit; the valves (22, 24) comprise: a first valve for controlling flow to or from a higher pressure point on the refrigerant circuit, and a second valve for controlling flow to or from a lower pressure point on the refrigerant circuit; and characterised in that: the second valve is a valve with a controllable degree of opening and the refrigeration system is arranged such that the degree of opening is variable whilst the refrigerant fluid is being transferred from the buffer tank to the main refrigeration circuit; and in that the refrigeration system is arranged such that, during operation of the refrigeration system, control of the degree of opening of the second valve is used in order to control the amount of liquid refrigerant fluid at an inlet of the compression device. the refrigeration system includes a buffer tank attached to the main refrigeration circuit, with valves (22, 24) for controlling flow of refrigerant fluid between the main refrigeration circuit and the buffer tank; the refrigeration system is arranged such that the valves (22, 24) are controllable to transfer refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of sub-cooling in the main refrigeration circuit; the valves (22, 24) comprise: a first valve for controlling flow to or from a higher pressure point on the refrigerant circuit, and a second valve for controlling flow to or from a lower pressure point on the refrigerant circuit; and characterised in that: the second valve is a valve with a controllable degree of opening and the refrigeration system is arranged such that the degree of opening is variable whilst the refrigerant fluid is being transferred from the buffer tank to the main refrigeration circuit; and in that the refrigeration system is arranged such that, during operation of the refrigeration system, control of the degree of opening of the second valve is used in order to control the amount of liquid refrigerant fluid at an inlet of the compression device. the second valve is a valve with a controllable degree of opening and the refrigeration system is arranged such that the degree of opening is variable whilst the refrigerant fluid is being transferred from the buffer tank to the main refrigeration circuit; and in that the refrigeration system is arranged such that, during operation of the refrigeration system, control of the degree of opening of the second valve is used in order to control the amount of liquid refrigerant fluid at an inlet of the compression device.