Laying gravel on top of existing concrete

It all depends on how uneven the concrete is and does rainwater run off it ok, if its even enough for your gravel to stay put once you put it on then thats a fine base for gravel for a back garden, providing rain water is running off ok. Do check your damp course level though on your house as well, ideally you don't want your finished level to be higher than a brick below your damp course. If weeds are coming through the existing concrete and providing your levels are ok then the only thing i would recommend you put down is a geo-textile membrane to help stop weeds coming through! Then your chosen gravel on top and level off with a rake.

It would be best to take out the existing concrete if this is uneven , then lay down a sub base layer , whacker this down followed by a membrane and then a good thickness of your chosen gravel levelled out and raked.

if you put gravel on a uneven concrete it may look OK for a while but soon it would look uneven again so the best thing to do is take up the concrete level it out and lay membrane and finish with your gravel stones kind regards D&G construction