Advice on building a raised patio

[Muckylad]

What a great website this is. I’ve spent some time trawling through it and feel that I’m starting to get a much better grip on paving etc. It’s inspired me to undertake something I’ve wanted to do for some time now and that is to build a raised patio at the end of my back garden. However, I was hoping you could offer some advice. The height of the patio will be 300mm above the existing ground level which is currently lawned. For the retaining walls I was planning to dig down 300mm to the subsoil and pour a 400mm wide by 150mm deep c20 concrete footing and then build the wall up using breeze block laid flat (ie 225mm wide). The patio area within the walls would also be dug out to the subsoil (again 300mm down). The sandstone slabs are 20mm thick and I’m presuming they would sit on a bed of mortar say 40mm thick. This leaves me 540mm to fill to get to the desired height. Can you advise on how best to do this. I’m thinking I would need 150mm of DoT Type 1 (laid in two layers of 75mm and compacted between each layer) for the mortar and slabs to sit on which still leaves me with 390mm to make up underneath that (between the subsoil and the Dot Type1 layer). What would be the best material and method to achieve this. Do I need another 75mm layer of DoT Type1 on top of the subsoil and compacted and then 315mm of DotType2 laid in two layers of 157mm (compacted between in each layer). Do I need to consider drainage other than the usual fall on the finished slabs and is there a need for geo-textiles or membranes. Sorry for all the questions but would really value your input.