What key blank do I need for the ignition switch on a Reliant Scimitar GTE ?

Having recently bought a 1978 Reliant Scimitar GTE SE.6A, I wish to have a spare key cut for the ignition. The snag is that my key cutters need the name of the manufacturer of the ignition switch barrel. I thought it was probably a Ford part, but the Ford blank they cut doesn't work, so it's probably not the right make. Any ideas? Thanks!