Update on this...
Having washed the starter (twice) as per Ed Wood's instructions, I followed the twice-daily refreshment, 50:50 F/W regimen to no avail; there continued to be some brief, thin bubbling, but no rise.
Following the advice here https://food52.com/hotline/18365-why-isn-t-my-sourdough-starter-active-yet-after-3-weeks
the starter eventually reached a useable state by refreshing once every 24 hours, with a slightly higher ratio of flour to water (initially 60:40, later 55:45). There were sure signs of improvement after 24 hours; after 5 or 6 days, the starter began doubling.
Temperature of proofing box was for the most part maintained at 23C.
When not in fridge, the starter is now refreshed every 12 hours, first at 55:45, second at 50:50; this to prevent it getting too thick.
First Margherita today using 20% starter; not a huge difference in taste (had previously used a 20% 'sponge'). From what I've read here, the CAMALDOLI culture is quite mild.
Hope someone else finds the link above as helpful as I did