Went to work for a hire company the other day, the job was meant to be simple, driving the tractor units only from depot to depot. Anyway this was changed as someone told me that you cannot drive a tractor unit on a class 2 licence 
, as the vehicles are plated for 40t , is this correct? as some where saying you can and others you can’t…even more !!
Anyway ended up being the driver of the “ferry vehicle” a 7.5t flat bed, still a good day though 
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First thing to consider is that there is no such thing as a Class 2 licence. A Category C licence is for vehicles over 3500kg and a C+E is for vehicles over 3500kg with a trailer. A solo tractor unit can be driven on a Class C licence and the plated weight has nothing to do with it. The weight thing is a throwback to the days of Class 1, 2 and 3 licences and is no longer applicable.
There was another long thread on here recently about this and you can find it HERE There is also a shorter thread, with a reply one member received from the DVLA HERE.
In order to drive any large articulated vehicle combination a driver would normally have to hold category C+E entitlement.
An “articulated goods vehicle” is defined as a motor vehicle which is so constructed that a trailer designed to carry goods may by partial superimposition be attached to it in such a manner as to cause a substantial part of the weight of the trailer to be borne by the motor vehicle, and as such, large vehicle entitlement category C+E is required.
However, in the absence of a trailer the vehicle would not conform with the definition of an "articulated vehicle and may be driven by the holder of full category C entitlement.
I hope the above information will be of some assistance to you.
The email i received from the DVLA a couple of months ago. 