I was wondering if there were any thoughts on my soil pipe and the likelihood of cutting it and finding a suitable fitting - to eventually take the 110 plastic system.
I've never seen it and judging by responses, nor has any of the plumbers on here.
As I suggested previously, I'd be taking it apart at the current spot where it converts to plastic and examining how they did it originally. This will also give you the ID and OD dimensions which will help you search for a suitable fitting.
I do wonder why it converts to plastic at that point! Was this a later modification to provide a vent? If not, why was it not all done in plastic at the time, if this was available?
Who knows... It would have been down to build spec of the whole development at the time.
If the transition from metal to plastic soil has been done with sand/cement or some other infill medium (as shown in your photo), its worth noting that this is above the wet side of the system (dealing only with air) and may not be/have been a suitable joint for lower down where it would be subject to fluids.
Do you have a local independent plumbing shop? It might be worth popping in there and seeing if there's an experienced "hand" behind the counter... You can then ask him about "odd metal soil pipe, used in the area" and if he knows how others have dealt with it.
There's a small estate round here where all of the water mains (including up into the loft) were done in alkathene/vulcathene (can't remember which) and only one little, out of the way, plumber's merchant stocks the fittings for it! Changing ballcocks was a particular issue until I discovered this local treasure
Mcalpine do a drain connector but it's for 110mm to clay or cast and you need to know the ID of the metal pipe you have to determine if it will work...
https://mcalpineplumbing.com/plasti...nectors-roof-flashing/dc1-blc-drain-connector
If your soil is larger then you may have to look at flexiseals or ferncos that will slot over the OD and get clamped down with a jubilee clip.