we run 4 by 2 units with tri axle trailers 99.9% of the time but we have one tandem [twin] axle trailer, i had it last night and one of our lads was telling me if i got stopped id likely get a 400 pound fine as were only taxed for 4 by 2 units and tri axle trailers, he said the tax for tri,s was less than tandem , about again 400 pounds.
is he right , is it ilegal to use a tandem when taxed for a tri, what would you have to do have separate tax for the twin or what 
ta
The other driver was wrong as it is no longer the case a vehicle is just taxed a 2+3 or 3+3, it is no done in bands. What he told you used to be the case up until the whole vehicle tax system was revamped. If, for example, your trucks are taxed in band E - £1200 - they can operate up to 38,000kg with a 3-axle trailer but can also operate up to 34,000kg with a 2-axle trailer or 26,000kg with a single axle trailer.
You can fimd the details Here on the DVLA site.