Replacing soil stack flashing

[Brannan]

I have noticed a large water ingress in my roofspace and find that it is due to water leaking in around the soil stack.

The Soil stack is internal and it emerges on the roof through a flush lead sheet. The stack itself comes out through the lower edge of a concrete tile which has had a cut made into it for this purpose. approx half the diameter of the stack is therefore cut into the tile and the other half just emerges from the lead sheet

This means that there is no weather collar attached and the space around the stack where it comes through the roof tile has been filled with mortar. There seems to be no sealant or weatherproofing around the lower edge of the stack through the lead sheet and the sealant question seems to have been addressed by glueing (!) the stack to the underside of the lead sheet, from inside the roofspace.

Its obviously been that way for many years without a problem but now I have to deal with a leak so I was initially just going to mortar and mastic everything back together but the area is in poor condition and this would just be a short term fix so I am now thinking that I will take the adjacent roof tiles off and slip a whole new lead tile (with an integral high collar) over the existing fixture and then mortar that back in as above and dress it all to fit. That way I dont disturb whats already there (as its all VERY firmly fixed in place and I am worried I might damge the plastic stack if I start trying to remove whats already there). Does this sound feasible?

Cheers. ANy advice welcomed