There is no specific distance mentioned in law however the legislation is clear that "other structures serving the purpose of walls" can be considered when determining if a structure is "enclosed or substantially enclosed".
The term structures can include natural things such as hedges or man made constructions such as brick walls. Structures can also be permanent or temporary and would therefore include things such as screens and large potted plants.
As structures can provide a sheltering effect any other wall or structure that is in close proximity to a smoking shelter may be considered as moving the perimeter of the area where smoking is allowed and the calculation would be to that extended perimeter. There is therefore a greater chance that the area would not comply and smoking would not be allowed.
It is recommended that shelters are placed as far away from other structures as is reasonably practicable and each situation will need to be looked at individually.