Correct, but in cases such as no seat belt or mobile phone, the constable has seen the offence and it is upto the driver to provide reasonable excuse, or exemption.
my point re the fuel is, a revenue inspector will dip the tank and find red diesel. No receipt, No defence, unles you bring the supplier as a witness to court who will give evidence that he supplied the fuel to you with duty paid
It all seems very unlikely but we all know how persistant the government can be when following up cases of tax avoidence, a private company probably wouldnt bother to go through the expense of court etc