On Wednesday, 20 September 2017 08:25:13 UTC+1, Harry Bloomfield wrote:
From what little I remember of our cast iron system, which was replaced
decades ago...

The socketed joints between sections seemed to be sealed by pouring hot
lead in. Brackets were fixed as described by Bob above.

There is nothing rigid in the joints, they have to be free to allow for expansion.The sockets were filled with gaskin and putty.
Gaskin = asoft rope.
The pipes are suspended from the cast in brackets; it's important sections do not rest on the one below so they can expand.
If you don't do this the pipes will crack.
The original installations were done from the top down to enable this.
It will be an angle grinder job to get the heads off the fixings if they won't prise out.
Once the fitting is off, you can get the remaining shank out with mole grips or a pipe wrench. They are usually hammered into wooden plugs