Thanks Pentaxuser, I'll check the wattage of the bulb to make sure it is correct. I checked the negative again and whilst I think it is a bit thin, I still think it is a reasonably exposed/developed image. You can see a poor quality digi cam shot here: http://dl.dropbox.com/u/1938344/20120716_220037.jpg It is held against white paper which probably makes it look a little denser, but hopefully you can see that there should be a usable image there.
As the enlarger is new to me, I checked all the lenses (it came with four) and put this one on for the printing session. It definitely stops down correctly and you can see the change in illumination as it does so.
I am also sure that I didn't leave it on f4. This Schneider lens has a useful leaver which allows you to easily flick between wide open and your chosen f-stop for focusing and after leaving it wide open at least once during my training course, I definitely checked that it was set back to the correct f-stop. I'm wondering if it is the wrong bulb.
If it is, are enlarger bulbs still easily available?