However, there was no finding that it
> was necessary for the disease, per se, rather than only for(theoretically
> separable) increments in severity, under Rob's theory, in which case the
> defendant should only be liable, under the but-for test, for those
> increments?
>

Yes. The question of quantum of liability was not before the court.The HL
in Bonnington Castings made no apportionment because the only quiestion
argued before them was one of liability, not quantification. When the
question of quantification has been argued in equivalent cases of
increments in severity, an apportionment has, rightly, been made (eg
Holtby v Brigham [2000] 3 All ER 421).

Nobody would still be citing Bonnington Castings if it had not been cited
for a proposition for which it did not stand in McGhee.
Rob