Patio pointing is wet and sandy when it rains

Peter is right the pointing needs to be removed and re-pointed. Be weary of the pointing mixes that come in tubs there are many different types, some you sweep in then hose down, I've yet to find a brush in pointing system I would recomend. I use a product called Parex standard mortar that you mix up and inject into the gaps with a gun then trowel off. It comes in a range of colours it's very strong and waterproof so doesn't crack in winter when it freezes. It is expensive but is fairly quick to apply once you have the knack so saves on labour and lasts Alternatively the old fashioned way Is often the best. A damp mix of fine sharp sand and cement, pushed and compacted into the gaps then trowels off. If the mix is wet it will stain the stone.

Remove and re point with proper pointing grout that is bought in large tubs from builders merchants for this type of paving slab, go to one, I know Jewsons sell it and ask the question and they will put you right Peter