The final task on the coach sides was to add the additional grills to the side of DC975280 following the usual approach chain drilling holes around the edge and splicing in a new grill cut from a pikestuff door.   With that done the two coaches were primed, with 975280 this was pretty straightforward with a good result.  With 975081 it was a little more difficult, while the coach side looked in a good position as soon as primer was added it showed up pretty much every joint needed rework.  In particular slight dips in the filler, these were resolved by adding a line of superglue over the joint which was filed back 24 hours later.  

With the support coaches done it was time to start on the actual optical car, while there is a 3d printed kit available for this vehicle it bares so little resemblance to the prototype it is of no value as a basis.  I think Britannia Pacific Models also produced a RTR version, however this appears to be only for the Network Rail version (and is very expensive), so the most sensible route is converting a Bachmann VBA as per Mick Bryan's posts elswhere on the forum (and in Update).   I dont think an accurate drawing is available, so I have been working scaling from a number of photos on Fickr.  I have deviated slightly from Mick's approach and instead of using the VBA body (with the doors filed flush / holes filled in) I have used just the roof on a plasticard body.  So far I have only prepared the chassis and the first end, the next is to follow this evening before I need to decide upon how I am going to model the centre narrow body...