Garden Workshop Base

[Gossamer]

Evening,
I'm about to lay a concrete foundation for the above and wanted to run things past people who know more than me...
My methodology is:

• Shutter the area with timber and stakes.
• Tramp soil down (its quite compact and I think clay).
• Lay thin layer of builders sand.
• Lay DPM (and upsides of shutters)
• Lay thin layer of builders sand.
• Lay 4-5'' layer of concrete (5:1).
• I was going to then lay a course of bricks on top which to secure my shed frame (on top of DPM between brick and stud wall).

Seems reasonable (to me!) but do I need rubble under the first layer?