Damp on outside wall...?

Question:A friend of mine has found damp on an outside wall. This is located where the outside drainage pipe joints are (in 3 places) so it would seem to be prettry conclusive that its caused by leakage at the pipe joints. So my question is: what's the best way to cure this...bit of mastic? or is it likely to require more than this? Do you think its just due to a bout of recent heavy rain overloading the drains a bit maybe? (he's in Scotland) and will dry out again? I think the drains are plastic (its a new-ish building) but I'm not sure.

Answers:
Have u checked that the gully is clearing
Caulking the joints wont help, if the system gets overloaded
(Surchanged) again
has the system been designed correctly-(sized)