Cinder - Suitability for laying a slab on

[rootsnall]

I read your write up on cinder and have a few more questions if you don't mind. There is no obvious forum so I thought I'd put it here. I have an old raised patio ( with 3-5ft retaining walls around it ) with 2x2 flags that have moved around quite a bit over what I think is about 45 years since they were laid. I have finally got round to renovating it and the first project is to dig a hole for a pond. This will be against a 5ft retaining wall on one side, on starting digging I was surprised to find it is 6 to 7ft deep in cinders, with a single layer of bricks and then sand on top of it before the flags. I have dug down 5ft and intend laying a slab to then build up the pond walls in blockwork. I was going to lay broken bricks and some MOT on top of the cinder but I'm now thinking the cinder itself is a better sub layer. Do you agree ? I have noticed it's properties when dumping it, you can run a wheelbarrow over it and it doesn't budge. The second question is I will then want to lay either Indian stone or block paving over the remaining old flagging. It would be a huge effort to dig up 6x9m ( x 2m deep ) of cinder. I am thinking Indian Stone on a substantial bed of mortar straight on top of the old flags might be the best option ? Or laying blocks on sand over the old flags ? Any thoughts much appreciated.