Hello I recently took the flags up to find hard core I wanting to turf the garden would I able to place top soil on the hard core or would I have to remove all the hard core need help
Depends on the hard core. At one time we started with hard core under lawns as drainage, broken brick as a rule with maybe a layer of gravel then top soil and the grass. That has gone it seems, most new builds get six inches of soil on a hard pan and the grass rolled out on that. To us old gardeners it has no chance though some prevail. If you have loose hard core or can loosen it up for free draining then there is a chance it would be OK. If it is solid then it must come out. Most will say take it out, I would try to loosen it up first.
I should have said if it is concrete then it must come out