floating floor on flags, insulation

customer wants to insulate on top of old flags but ony have 40mm to play with as low ceilings. doesnt wnat to ruin flags as good examples and maybe reuse one day.
was going to lay dpm25mm celotex then 18mm mr chipboard. but took another look at job and in one area the flags are quite uneven and may cause bounce. they want to lay cheep laminate on top as finish. any ideas? is there a good insulating rolled uderlay i could use say 10mm thick and poack floor out with hardboard.???
cheers

 

customer dont want to pay for lifting slabs dpm concrete etc so have to come up with another option.