Happy to report that I managed to replace the display in my Agilent 34401A. The old display showed signs of having been used on 5 digits for the best part of its life, the new display is bright and evenly lit (although that is hard to show on a photo)
It took me an hour, most of which was spend in taking the front off and taping kapton to the legs of the new display. The threads on this forum about similar repairs helped a lot. Thanks guys!
The experience I got from repairing slot machines in a previous life paid back today. A solder sucker is like a bicycle, once you learn how to use one you never forget it