UPDATE:
got it working, I attached to the head tube in the end - I realised that in order for the basket to clear the surfboard the surfboard would have to be so low that I ...couldn't really turn right (I didn't realise this for ages because I was working on the bike with it leaned up against a wall, at an angle....duh, schoolboy error....)
I angled the board slightly so it's closer in at the back and a little further out at the front, which gives me plenty of room for steering - I just have to remember that I don't have full steering range when doing slow maneuvering, at normal traveling speed it's not an issue.
I took the rubber covers off and used the tightening screw on the rack mounts because my boards are on the heavier side, not great for the paint (on a brand new bike!) but it's a utility bike so I'm happy with that. Also anti-seize...
All in all it works great, the pack rat is a fun speedy ride that now also carries my surfboard (wetsuit etc go in the front basket). And the moved-by-bike rack detaches in seconds so is very convenient to pop on and off. Perfect!
