I am fitting a toilet and have to join into the old cast T piece. I cut the old pipe out and the lead but now have a connection of Id 125mm. I want to connect 110mm plastic into it but all the adapters seem to have rubber fins at 115mm. Does anyone known of a product the right size?

If fitting into the T you will need a "collar" of the old cast iron in there first. Well, that is how I did it.

Yeah I just starting to think that. I have a short straight length that went thru the wall. I'm thinking cut a ring off that and bond it in with sealant then adapter for the plastic. I can't find an.adapter that goes straight in. What did you use to fix the collar in.

you get a rubber coupling that adapts iron to plastic, plumbers at work call them timesaver couplings, not sure if thats a trade name or generic name for them. you may have to cut above and below and add a plastic tee.
most builders merchants plumb centre etc will have them

Whats the OD of the cast pipe?

It's 4" but I removed it from the T and all the lead that held it in. The socket I'd 125mm I'd. I wish I'd cut the pipe flush now, then I could use the adapter they have in wickes and elsewhere.

Is this what your after ?

http://www.toolstation.com/shop/Roofing+%26+Drainage/d250/Soil+%26+Vent/sd2849/Flexible+Adaptor+Clay+to+PVC/p63345

I cut the pipe and left a few inches in the T!
A bit of silicone or whatever should hold it in place though.

Mark s no unfortunately I need a 125mm internal and 110mm external.
I'll do what cliffhanger suggests I think.
Cheers pete