Thanks Dean - I used Aspire's default step over, so no idea what it was set to! I'll look into it tonight and get it into that range.

Out of interest, what would you expect to see DOC wise on a router like this with a 4mm single flute? APT suggests slowing down from those parameters once you're over 1.5 D, but I assume that advice is really for mills; 6mm felt like quite a big chunk to be taking.