I've already found a chassis for it.... it looks like it'll turn up with a B12 chassis (driver spacings 7'3'' and 7'3'', vs. prototype spacings 7'3'' and 8'3''..) 

However it transpires that a J52 has the same coupled wheelbase.  Now the Hornby J52 has a slightly out-of-scale spacing, but that is something I can live with.  The J52 uses the same chassis as the Hornby 'Jinty', and I have one of those in my spares box.  So i'm thinking of using the Jinty chassis with the B12 wheels and bogie....

7'3"x 8'3" is the same as a GW Pannier, or Gresley V2. I think some LNWR engines also used this wheelbase....

A B3 is a difficult engine to portray, and I think you've picked a real challenge with your sample. It's not just new chassis and new cabsides, but also a new tender.

I don't think anyone has dared to produce a kit yet (although Great Central Models are thinking about it).

It's a challenge indeed (though one I'm really looking forward to!) 

I am looking at picking up either a V2 or a Black 5 chassis to use, if I can't find one of those two I'll look for a pannier (which I think, though not entirely sure, uses the same chassis as the J83/ O8/ Jinty?)

Of course, if it turns up on a Black 5 chassis it would be great!

A Black Five is 7' x 8'. May look a little short once you mount the full 6'9" drivers. I might have a drawing you could use....

I've got a 4mm Charles Reddy drawing, but that only shows the side elevation- if you have a 'proper' set of plans it would be quite useful, thanks!

The loco arrived this morning, and to be fair the chassis (I think it is the Black 5 chassis) doesn't look at all that bad.  Not right, but not so wrong as to be immediately obvious.  I switched out the Black 5 bogie with a spare B12 bogie I had lying around and this immediately improved the look of the thing.  The cylinders are set a little too far back bt to be honest I don't think there is anything I can do about that (and as the cylinders sit above the centre line of the smaller B12 bogie it 'looks' right even though it isn't).  I think the only real alteration I could make to the chassis and be confident it would be an improvement would be to replace a pair of huge plastic 'things' masquerading as motion brackets. 

Other than that there really isn't much wrong with the model at all- too few boiler bands and handrail knobs, the wrong buffers on the loco and a bit of cab roof which shouldn't be there.... a bit of care and some decent parts and I'll have a very respectable model.

I have a single GA drawing of the near side of the engine. It has been reproduced to the weird scale of 6.4mm/ft in order to fit within an A4 page, but I'm sure you can scale it down to 4mm/ft.

I'll try a scan of it. Let me have your e-mail address.

UPDATE: right, e-mailed it all over.