After having fitted a soil heating cable with a probe thermostat and removed the insulation and fan heater, and then nervously watched as the temperatures continued to plummet, the orchids in the plunge bed have survived and are now starting to develop nicely. Except for the Anacamptis papilionacea, which is still chlorotic!
This advice was gleaned from the HOS forum, and I still think it was the right course of action for other reasons (humidity control, cost, expandabilitylight levels) but it doesn't seem to have solved this specific problem. Interestingly two other replies suggested that removing the bubblewrap may have been a mistake (too late unfortunately) and that they suffered from the same problem. My problem even made the Laneside Orchids Newsletter - fame at last! Interestingly Jeff Hutchings uses bubblewrap insulation and similarly has the same chlorosis issues with some species, and suggests using lights. I'll have to get in touch to find out more from him.
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