Garden project

Got a bit of a project going on at the moment, Im lifting an old concrete slab patio that tends to flood and replacing it with indian sandstone. The concrete flags will be reused by myself and anyone else who needs them.
My plan is to raise the patio by a few inches and find the manholes that were buried under the patio by the previous owner, i will help the drainage by installing channel drains in the main patio area and allowing the rest of it to drain into a boggy area planted up with moisture loving plants.
Im hoping the base thats there stays in place when i lift the slabs but that will make it awkward finding the manholes, they wont be that far down so i plan to use a hammer and chisel to tap into the base and hopefully the sound/vibration will be detectable by listening to the soil pipe (sound will travel quicker through air in pipe) Hopefully this works and i dont have to unsettle the base with exploratory digging, ive ordered a couple of recessed manhole covers to place over the holes to allow easy access.
The flooding problem was the result of the builders removing all the top soil (and flogging it) leaving behind subsoil and impermeable boulder clay, ive had to import tons of soil conditioner to improve the soil and to raise the beds enough to stop the plants getting waterlogged in winter and baked hard in summer. Once the patio is done the bog garden will be the lowest point and the water will accumalate there instead of on the patio, well thats the plan anyway.
Anyone else with similar problems or projects ? I would appreciate any input and advice, espiecally with regards to laying indian sandstone, do you seal it first? or not at all?
I plan to set the height by putting in the largest recessed manhole cover and the channel drains first, is that the best way to do it?