I too am flummoxed, as was the chap in the shop when I took it back. They're going to give it the old once over, and I'll report back on what they find.
I should point out though, that my previous b&w work was on the 605 black and white head, and the problems with the dark projection only started when I traded that in for the colour equivalent.
By the way, I don't think the wrong bulb is in. I double checked with nova when I bought it that it was the right bulb, and the thing is so bright in the head - before you close the hatch and stick it back on the enlarger -that you can only stand to look at it for about a second. In addition, the negatives are all correctly exposed and developed.
Thanks once again.