Graham, currently across the tyres it is 38.83mm and with the 0.5mm added in frame width I have 40mm between inside faces, across the boss is 40.3mm, it's actually the spokes nearest the boss that are touching the frames and are actually preventing the tyres from shorting out.
As can clearly be seen above, the inner chassis is much narrower and allows a lot of side play for the wheels, too much, so I'm hoping that once that has been reduced (washers? ugghhh) the side play should stop and thus tyre contact with the frames. I'll aim for almost zero on front and rear axles and a small amount on the intermediate axle to aid tight corners, though the short wheel base and gauge widening should reduce that issue as well.
Frame attachment, The only part that needed flexing (couple of mm) was the base of the front ladders, I have not added the diagonal brace and step (4 & 3NS) or the front step brace (9&10 NB these parts seem optional on later dated machines).
I tilted the frames 45° to get my added angle iron past the frames (you may not have to do this) and tank mount then slid it forward 10mm and dropped the rear end down under the overhangs and slid back until it stopped, then dropped the front down between the steps and buffer beam (the buffer beam may have flexed a little in this process) with a nice tight click.
One word of warning there is a strip on the front buffer beam that is supposed to (I presume) fit inside the frames unless I have bent it wrong and it fits over the frames once inserted, I found these to be too wide so trimmed them back to the frame inner width and then the frames dropped in easily. You need to cut where there are red ovals to get your frames to fit easier, a width of 40mm or just under will suffice, I am just under so there is a small gap which I will fill with a small piece of angle (see more below).
In the last image you can see some of the extra work I wish to do, where the red oval cuts are I will add a short piece of angle to secure that plate to the frames, then I will cut where the red lines are to remove that section which has the holes in from the buffer beam pipe stands etc. This will be followed by a solid piece shaped accordingly which will fit under the retained plate and will thus form the lip the buffer beam makes with this part on the 1:1 loco, IE the real plate is not flush with the buffer beam lower edge.
Hope that makes more sense, if not fire away with the questions.
. Oh and it true WT spirit, pictures please 
Yes, it does have smaller wheels that the LMS variants should have had, but your average exhibition visitor didn't know any better, and it let us play dual periods on the layout which tended to lack any modern traction for 1947, or any steam traction for 1959...
