There's some more details in this
flyer for Network Rail Consulting. Obviously not to be believed in its entirety, but presumably the scope was at least right when they started.
It's no help on the phasing - but given how much detailed stuff is involved that could have been pretty well invisible. One point of note is that it does not say whether axle counters are the immunisation of choice, which I understood was true for the whole
GW» electrification project.