Reporting back. I mixed new developer, checked temperature (32C) and waited a good 5 minutes before printing to let my eyes adjust so I could check for light leaks. Here's the results (not so saturated in real life):



Top: original print made last week (105M, 95Y)
Bottom right: print made this morning (105M, 95Y)
Bottom left: print made this morning (70M, 55Y) (borders are bright white)

I then changed my bulb to a new one that arrived yesterday and the pattern was the same.

I decided to try a B&W print:



Top: print made a few months ago (170M, 0Y)
Bottom: print made today (170M, 0Y)

So, in sum, prints made in B&W seem consistent. In colour, I have a shift of around -30M, -40Y. I've been printing Superia 800 on this enlarger for over a month using a starting base of 100M, 90Y. Now that seems to have shifted to 70M, 55Y. Next I'm going to open a new box of paper and try a sheet. After that I'll look inside the enlarger head to see if there's anything obviously wrong (anyone have a scanned user's manual for a LPL C7700?).