Bit late to the party, sorry - but a bit busy packing up a certain location a few miles north along the line ready for its second show this coming weekend.
Anyhow... re the water cranes. I've been making some of these up for Grantham and, being of GN origin, I have been using the Mike's Models ones that it looks like you have used. What I've noticed from study of photographs was that they differed in both height and reach according to the specific location at the station/yard/depot.
What I have done here is to mutilate the kit (left) compared to it as supplied (right). Some cranes only used one of the swan neck extensions or in fact none at all. And the length of the arm only appears to have been as long as it needed to be in terms of the distance away from the track(s) where the locomotives were stood. This one is actually the platform mounted one for southbound trains (fortunately well photographed - picture below from the ever-excellent return to Grantham website)
You can see here only one swan neck extension piece (very probably added when the pacifics arrived as you say - previously it would have been OK for Atlantic tenders!) and the noticeably shorter arm. The octagonal base is either lost altogether or buried beneath the platform surface.
Of course, without sight of pictures of the actual cranes at Peterborough, I don't know quite how relevant the info above is(!) but I hope some of it is useful.