Some general pointers.
TF houses with a block outer leaf require a ventilated cavity to maintain a dry structure (the tf is structural). Your surveyor has highlighted a possible issue with this ventilated cavity i.e. there is not sufficient ventilation.
The ventilation deficiency can be readily addressed. However, the obvious risk here is that there is already some moisture damage to the structure but the only way to ascertain this is to do some exploratory invasive work which the vendor is unlikely to allow.
So the question in my mind is how long has this issue existed and how likely is it that rot has already set in.? (How old is the house?). When you viewed it were there any visual issues or damp / musty smells? Is your house part of a scheme and if so are there differences in cavity ventilation between yours and your neighbours. What ventilation systems are present in the house itself which might have relieved / reduced the amount of moisture available to mitigate the issue. What type of outer skin is on the house (rendered blockwork, unrendered brickwork, other?) and how exposed to the elements is the house?
So more questions than answers I'm afraid but if under-ventilation has existed for a time these are the questions that need to be answered to help come to an educated decision (guess) on how or indeed whether to proceed. Perhaps your surveyor can help you further with this.