An update a month later...
I've gone through several ideas - refitting the damaged treble valves, rearranging treble valves from higher up the keyboard, buying new wood valves. But I have to report excellent service from Hohner. Failing to find spare parts from dealers, I contacted Hohner and received an email from the same chap who deals with Hohner certificates. I provided a photo of the damaged parts (Hohner call them treble valves) and I can order them direct, 1.8 Euros each. So excellent service from Hohner. Although I note that wax is not considered the best material for plastic valves, I'll fit them with wax in the same way as the instrument was constructed.