Lucy:
They used to be transferrable. We just had them in the key bags for the runs when I was on New Look, so whichever motor was allocated took the bag. Same with Severn tags.
Not quite true with Severn tags. Severn tags have two types:
“Trip” or “Fleet” tags record each crossing and subtract the cost of the crossing from your tag account balance. These tags are transferrable between vehicles of the same class.
Season tags give you unlimited crossings over a fixed period (month/year). They can only be used for the vehicle whose registration number was specified on the SRC form.
pugwash:
Can anyone tell me if these dart tags are interchangable between vehicles or are they fixed?
i.e. Can one truck go over one day and then give another truck the same tag the next day? and
Are they registered to a specific registration number?
As I said in an earlier post,our Dart tags are not unlike a mobile phone,and we have a number in the office.If we are given a run where we have to use the crossing,we have to sign out the tag,then hand it in after the trip.
Yup as above …
And remember , on the approach to the booths take no prisoners …
Look around you at the booth and you can see other lorry drivers eyeing up your truck to see if it will be freighted up or not and see who can get the away and gone 
It’s like a truckers gumball rally .
Can anyone tell me if these dart tags are interchangable between vehicles or are they fixed
They are interchangeable between comparable classes of vehicle.
Ie.
Car
2 axle rigid
multi axle.