I fixed my issue with the eBay actuator. As the part I bought was for the R107 vs. my W114, I had to modify the arm a little: drill a new hole and cut the end of the arm off (with tin snips / easy).

However, while the rubber diaphragms on the "new" part hold a vacuum, they are old. I'm sure I'll revisit this in a few years' time! (In hope of prolonging their lives, I liberally applied Gummipflege.)