Thanks everyone, I will have another go soon. The instructor on the course suggested aiming for a base exposure time of about 12-15 seconds to allow time for dodging, so that was what I was going for. I may try a different image next just so I don't go crazy, but I'd like to come back to this one and crack it. Maybe it really is just massively under exposed, but like I said, it scanned fine.
The paper is fresh, bought new from AG-Photographic a couple of weeks ago. I did make the stupid mistake of turning the light back on without completely closing the box, but I developed a whole sheet and there was only the faintest bit of fogging on one corner of one edge and it only extend about half a mm into the page. Hopefully that will be the only time I will make that mistake.