I have looked at some pictures and I think you are correct. It does make a lot of sense too.
The conservatory/orangery does not get damp as such, but is knocked through to the kitchen and dining room which means there is a food source and a lot of humidity. There is underfloor heating giving warmth. The insects are common place on the table where there are bread crumbs etc..
We have been reading a collection of old material that is 30 to 100 years old from a private book and postcard collection which might have introduced them.