hi there, OP here!
just to close this topic ...
after trying many different methods, I managed to get my hands on boric acid.
I mixed 1 spoon of BA with 3 spoons of sugar, added water to get it to a thick, syrupy consistence and put it on aluminium foil on the tracks ...
after 1 hr. the alum. foil was black with ants ...
after 1 day (and some free refills) the ants got scarcer and scarcer and now after about 40 hrs my house is really ant free for the first time in months (I am not too worried about the odd ant I keep seeing)
no more black trails in different rooms, etc...
so boric acid and sugarwater did the trick. quite cheap too, about 5 bucks for a 10 year supply of BA.
just thought I'd share this information
cheers
al