As far as I know you don't need 500 hours, I know of a 200 hour merchant with an IR who just started as SNS co-pilot
Also low hours does not at all mean you won't get hired as a low hours person is a lower cost than a high hours one.
OGP manual appendix 5B suggests 200 hours is Ok, appendix 5A suggests 500 hours but is only a recommendation I think. See page 118/120
http://www.ogp.org.uk/pubs/390.pdf