110mm plastic soil pipe into Timesaver connector

[space refurbs]

I've had to cut out a vertical section of cast iron soil pipe and replace with plastic. I only had to cut the old cast iron once as the bottom part went into a Timersaver straight connector collar thing that is built into the cement floor, and I just levered it out.

I have inserted the plastic into this collar (which has a rubber seal inside it) but it's still slightly loose and water from the soil pipe comes out. I can't tighten the collar as it's cemented in - can I use ordinary silicon sealant to seal the join, or is some other sealant better, or should I try some sort of rubber seal in there?