PVC into Cast Iron

Looking for some advice! Found that the waste pipe from our bath was leaking where in inserts into the cast iron pipe going outside.

Have taken this apart to inspect and whoever has had a go previously has tried to pack out the gap with silicone which has failed.

The cast has an internal diameter of around 50mm/2 inches. Whats the most secure method of making a join between this and the 40mm waste pipe?

That looks as though it could be a threaded socket ? if so you can get BSP threaded waste fittings as per this picture.