I am afraid might be no easy answer to this. Firstly, where are the gaps? top or bottom of slope. How big? How many? It is presumably laid on sand and I am guessing that the gaps have been exacerbated by the growth, and removal of weeds. True, sand could wash away but, you could do this anyway and hire a wackaplate for a day (about£40.00) to vibrate the blocks into the sand. They tend to bind together. There are also some mediums on the market which work like grout - you brush the dry material into the gaps and it sets within 90minutes (Geofix) but, this always looks a bit weird with paviours and is not cheap. Grout Sand and cement may look even worse but you can, at least, add colour to the grout mix. Let me know more info about the gaps and I will have a think (We are landscapers).