1. An apparatus comprising: processing circuitry to perform, in response to a vector instruction, a plurality of lanes of processing on respective data elements of at least one operand vector to generate corresponding result data elements of a result vector; wherein the processing circuitry supports performing at least two of said plurality of lanes of processing with different rounding modes for generating rounded values for the corresponding result data elements of the result vector; a first control storage location to store a variable rounding mode parameter indicating a default rounding mode; and a second control storage location comprising a plurality of control fields each for storing control information for controlling a corresponding lane of processing; wherein for at least one type of vector instruction, the control information includes a rounding mode value specifying the rounding mode to be used for the corresponding lane of processing; each control field includes information identifying whether the corresponding lane of processing is as an active lane for which the corresponding result data element is to be generated in dependence on said corresponding data element of the at least one operand vector, or an inactive lane for which the corresponding result data element of the result vector is independent of the corresponding data element of the at least one operand vector; and in response to said at least one type of vector instruction: when the rounding mode value stored in the second control storage location corresponding to a given lane of processing has a default value, the processing circuitry is to perform said given lane of processing using the default rounding mode indicated by the variable rounding mode parameter stored in the first control storage location; and when the rounding mode value stored in the second control storage location corresponding to said given lane of processing has a value other than said default value, the processing circuitry is to perform said given lane of processing using the rounding mode indicated by said rounding mode value stored in the second control storage location corresponding to said given lane of processing.