100mm within my concrete slab would concern me structurally. Perhaps unnecessarily. I am going to get these which are super cheap. Also I should be able to get some nice smooth large radius turns using a blowtorch to bend them which I would not be able to do with a 100mm soil pipe.
I have just bought a house that has a gas fire with back boiler and want to get a combination boiler fitted to remove the unsightly fire and get rid of the water tank in the bathroom so I can fit a bath. I think a 24kw boiler should be adequate for this small 2 bedroom house with 1 bathroom...