Hi Mark sorry about last post 
we clean alot of tarmac never had regrowth (noticable) in that sort of period, q was what kit you used lts/min bar flat surface cleaner ??... It is hard to remove all spores due to the makeup of tarmac but there are afew targeted moss killers left on market which you can use after cleaning to eliminate remaining spores, but not Diy products. Generally less effective!
Usually apply via knapsack same old story need license as ur a contractor
Mark,
The above post from Mike, was a pretty informative post.
My post, originally was a serious post too. If i was having your problem, and it was a decent sized job, for a property management or corp i would definitely offer a maintenance clean, at a reduced rate if it was nearby, or if you knew you could still make decent money out of it.
The weed killer wont work straight away, it has to soak in and kill all the spores, as just washing the area wont kill the spores inside the tarmac or blocks. so basically you need to clean it, then add the weedkiller (gallup etc) and there may be some regrowth until the weedkiller takes effect.
Sorry if we offended you with our answers, but you were asked what kit you used, and you just never came back...