Hi Jamie, the MSA recently introduced a scheme to 'license' marshals so that they have a register of experienced guys. To get one only involves a training day, usually a dummy Rally (we had one at Kames two years ago, and the plan is to incorporate one/two into this series, where you will be taught safety, how to operate the Start Stop clocks, flying finish, checking in/out stage/service ect). Then marshal on three rallies (you can claim grandfather rights).
From then on Car clubs will be getting in touch to ask you to help and, your local (Wales !!) WRC round gets to be free.