We propose to test and pre-commission the recently constructed LET incident beam polariser. The device employs high powered lasers to optically pump a 3He neutron spin filter directly on the neutron beamline in order to deliver a constant neutron polarisation. The key objective is to use the existing CRISP beamline to provide calibrated values for the neutron and 3He polarisations achieved by the device. Comprehensive tests have been performed on the device in a laboratory environment, but absolute values are not possible without the use neutrons. It is imperative that these tests are performed before the long ISIS shutdown, so that any necessary alterations can be made to the device before final installation on the LET instrument.