So after a bit of experimentation, turns out its less the strength of the sodium hydroxide that matters, more the temperature. Using a water bath at ‘slightly warm’ (technical term :P) with a 1.5% solution, the board developed perfectly. This is after 8 minutes in the exposure box, but still, thats a 10mil trace its done. I think its 10mil anyway, will check my original drawing later. Just about to etch it to make sure its worked well.