Hmmm, fair question. The thing is, you can produce as much as you like, it's just that you can only use 2500 litres on the road. In the field, you can run on anything.
It would be very interesting to contact HMRC, and ask them about alternative fuels in tractors.
If the tractor is being used for agriculture, then it is allowed to run on red, within a certain radius from its registerd base. These boys hauling straw from hereford, up round here, are breaking the law. No one has been done for it yet. The spud haulers are technically breaking the law as well, although that has been thrashed out in court, and basically, the law backed off.
What if you were runing WMO? would you then be allowed to drive on the road with the tractor? I think you would. Its the way the machine is taxed, and the licensing needed to drive it, which defines what the duty rate is. You could tax a tractor as a lorry, and run it on white, and some do, because its not an agricultural tractor, its being used for haulage. Then you need a class 2 license to drive it.
Muddied the waters a bit.
