asked the Secretary of State for War whether local welfare officers are given facilities to visit detention barracks and camps and are allowed to interview soldiers privately?

The answer to both parts of the Question is, "Yes, Sir."

Are these visits impromptu, or are they very carefully stage-managed in advance?

I imagine that notice is given, although not necessarily very long notice. It may happen also that the detainee himself asks the welfare officer to visit him.

Can my right hon. Friend say, from his own knowledge, whether these interviews are frequent?

No, I could not say offhand; but I think they are as frequent as the detainees desire.