tetaining wall advise please

seabreeze_3

hi we recently moved in to our house we are detached the problem is next to the garage there is a retaining wall about 7ft high which runs down the side of the garage and in to the garden the gap between the garage and wall is approx 20cm there is a type of membrane between the wall and the garage wall . the bunglow next door is above us on this level we have a problem with water seeping in to the garage at the bottom of the brickwork where it meets the floor. the thing is looking down the gap and at next doors property he has concreted along the gap between the wall and the garage thus no gap and it looks like lumps of earth and concrete have fallen between the two would this cause the problem of the rainwater seeping in they are a retired couple in their 70's so we don't really want to rock the boat if the fault lies somewhere else ie the original builders the house is only 20years old so we could not call on the nhbc to get it put right any advise would be most helpful.
thanks