Hello, Taz.
I think the only problem changing 7034 to 5069 is that the former has the later (post-war 'Castle' batches), 'L'-shaped style of cabside grab-rail (what I call the Type 3), whereas 5069 had the early, straight, horizontal pattern under the cab side-window (what I call the Type 1).
Incidentally, the 'Collett Goods', or 'Baby Castles' to some, had three types of cab grab-rail: Type 1, straight, horizontal and directly under the cab side-window (generally modified to Type 2 on the 2251s); Type 2 (not used on 'Castles'), as Type 1, but with an extra, vertical rail in front of the window; and Type 3, the 'cost-engineered', one-piece job providing the functionality of the Type 2.
Your superb photographs of 7034 show the model to be a beauty. But I'm still sad that Hornby saw fit not to represent at least the lower jaw of the massive slidebar bracket. If anyone is unconvinced, just take a look at any decent picture of a G.W. 4-cylinder locomotive (for example Stationmaster's excellent avatar can be found at ...>Prototype Galleries>The Stationmaster Looks At Locos>7029..., 11th.October 2009), and tell me that you can ignore that hefty, cast 'hockey stick' protruding from between the adjacent, lower quadrants of the first and second driving wheels.
Moreover, Hornby have even correctly modelled the displaced front brake hanger/shoe assembly - displaced to provide room for that motion bracket's lower jaw! And although I haven't yet got my stickies on a new Hornby 'Castle', I am sure that at least the lower jaw could have been represened, even in 'OO', possibly moulded as part of the front brake hanger/shoe assembly or the chassis keeper plate. The vertical part may be more difficult, especially in 'OO', and is, anyway, less noticeable.
I hope the above helps, and thanks for your fantastic photographs of an impressive model. They have persuaded me to buy a few, and to seriously reconsider my 'silk purse from sow's ear' upgrade project on Hornby's previous offering of Dapol/GMR/Airfix provenance .
Best wishes,
BR(W).