Thanks again, really appreciate this conversation, very informative. So looks like it was coming in from both routes then.
Going off on a bit of a tangent here, more out of interest than anything, but I have often wondered why the cement skew starts leaking, and I presumed that movement eventually breaks the seal between the slate and the skew. If so, could that not be repaired with a grey flexible sealer like CT1 rather than ripping the whole thing out?
I mean presuming that the cope and the cement skew are sound, and both were painted with water seal, and the sealing edges of the cement skew were sealed with CT1, is there any reason why that wouldn't work?
Its obviously not as elegant, and probably not as long term as replacing the cement skew with lead, but could it work?