Thanks Ben. I'm not trying to replicate a specific flow on a particular date, but a fictitious, somewhere in central Scotland circa 2007ish type of layout (ie the EWS rather than DB Schenker era). The VTG Blue with Enviro markings are therefore a little too modern (although they look nice) as is the Greenenergy livery. However, the others all seem to fit my time period, although the plain grey VTG branded tanks would probably be most appropriate from your response. Looking at Flickr there seems to be plenty of photographs with a Colas locomotive (most frequently a Class 60), but few from the EWS era.
My wagon spotting skills are perhaps not up to scratch, but this photo on Flickr seems to show one of the VTG grey wagons immediately behind EWS 66200 at the head of a rake of otherwise green TEAs, so it would appear that an isolated wagon in a rake of older wagons is permissible. This photo on Flickr also seems to include a single VTG grey wagon in the rake behind DB branded 66080, although it is the second wagon in this case, while in this photo on Flickr, the first two wagons appear to be the VTG branded grey wagons heading up a train of otherwise green TEA . Unfortunately these are all 2015 photographs, so I guess even the VTG grey ones would have been rare in my time period.
Interestingly, these wagons seem to make up the majority of trains headed by Colas motive power and the red ones seem to be particularly common. Looks like I need to keep searching for justification.