there is some doubt over the benefit of erecting OHLE on some routes, thinking specifically the highland / far north routes/QUOTE]
I'd agree, but the numbers are so trivial - a couple of units on the Heart Of Wales Line, maybe half a dozen north of Inverness, a handful for the West Highland, one or two for Blaenau Ffestiniog... it's a trivial amount in the grand scheme of things
I agree with Yorkshire Bear, here: the solution is to vastly increase the amount of electrification in the UK
My view is that once we've wired the remaining "main lines" in CP5 (London to Swansea, London to Sheffield, Glasgow to Edinburgh, Liverpool to York etc) then CP6 will see a lot of the gaps filled in between main lines, which will see 75mph DMUs replaced (e.g. once the main Transpennine line is wired, the Calder Valley line looks do-able in CP6 - plenty Pacers and Sprinters there).
So once we get to the middle of the next decade, the need for DMUs comes down considerably - whilst CP5 may only replace half of the Pacers, we'll be able to scrap hundreds of DMUs after CP6.
Which begs the question - do we patch up the existing 75mph stock for another five years of work or do we spend hundreds of millions building new DMUs (to replace Pacers) for the sake of a five year spike?
Previously my opinion was that we'd need more basic 75mph DMUs (since it looked like
all Pacers would have to be withdrawn after 2019), but the noises about being able to upgrade Pacers and follow-on electrification has changed my mind.