Red diesel is a huge tax concession to poor farmers, bankrupt builders etc for use in their non-road engines and plant.
In case anyone is toying with the idea of running their diesel engined road vehicle on red, remember that HM Customs & Excise are not as ineffective or sensitive as HM Inland Revenue.
Evasion of duty, although just another 'tax fiddle', is regarded as a serious offence and may well result in the seizure and destruction of your vehicle as well as a hefty sentence.