Hi I would remove all of the stone as it won't compact like Hardcore. Over time the sand will mix with the stones and you will feel this through the grass .my advice is to remove the stones and lay a sub base of type 1 hardcore then your sharp sand this will provide a good solid sub base with good drainage I usually install a base with 50 mm of hardcore and 20 -30mm sharp sand .alternatively if you would like to save some time and money you could also consider using granite dust also known as grano .

Hi i would also do the job as the last comment said but i would lay a weed control under The sharp sand

Hi

I would also remove the stone as it is not compactable, put a membrane down and lay 75mm of type 1 hardcore, do not use sharp or kiln dried sand, it doesn't shape well at all and as such it will move over time and you will get dents in the lawn. Use granite dust at 40mm this is 4mm to dust and will give you a solid base that will remain for a number of years. Also you should frame the lawn rather than some companies that will trim and pin. Nothing wrong with that if you are dealing with curves but if you can install a frame.

If you simply apply sand over the gravel eventually the sand will wash through the gravel, although initially it will appear to look good a preform well it will be short lived meaning you would need to re fit and that In its self is costly. Do it right the first time and have long lasting quality.. Use a crushed stone sub base compacted approx 3 to 4 inch deep this will allow water to drain through but also give you a solid base, then use either stone fines or a grit/sharp sand on top compacted also and screeded off for that lovely smooth flat lawn finish.
Good luck with your project..

Remove the stone.

For 40sqm you will need
5T MOT
3T Granite (6mm-dust)
Kiln dried sand to infill.

Any questions pop me a message

Jez.