Finally found time to investigate this and it turns out that the gasket was kinked. This in itself wouldn't necessarily be a problem but it had also developed a hairline split - presumably because it was under tension.
To my knowledge, the rocker cover hasn't been disturbed since new so I guess it was sloppy assembly at the S&S factory? Or maybe a batch of out-of-spec gaskets? I think I'll check the other side just in case...
