Another misunderstanding then.
I find this stuff incredibly complicated.
Routing is:
INV - LHR - JFK - PHX - SFO (40 / 140 / 140 / 40)
SFO - ORD - JFK - LHR - INV (40 / 40 / 140 / 40)
The change is on the ORD - JFK section. Leaves me less than an hour for transfer after the SFO - ORD flight, which I'm not particularly comfortable with to be honest (I know a lot of you guys manage insanely quick changeovers, but I don't really do this sort of thing much).