I am going to hazard a guess that a lot of the delays would be down to paper work issues. Balfour Beatty pretty much packed up their gear and left with about a months notice. I am sure it was planner or Joseph Locke who also said they hadn't finished surveying the line and what little they had done they didn't share with the next company, hence all the new bases etc. Then there was the issue with the European directive on wire heights.
I don't really buy the Balfour Beatty issue.
They are still hard at work for NR on Crossrail GW wiring, so NR can't have a policy of not using them.
I believe they are also still engaged in some way on the NW project (design?).
So the year's hiatus seems to have been pointless.
Contract disputes usually have two sides, and we've only heard the NR side (unofficially).
There is a big contrast between the Bolton and Fylde route electrification in terms of PR.
Fylde is getting the whole PR treatment (first pile and all that), while the Bolton work has never been publicised or explained.
Meanwhile the tunnel and arches work, and now Ordsall Chord, have had the full treatment.
Fylde also seems to be working weekends and making use of all the engineering possessions, while Bolton seemed to slumber on while weeks of possessions passed without any electrification work.
No doubt there is an explanation.