Is it an offence to display hazard diamonds on vans?

Many operators of vehicles that are exempt from the requirements to display the orange plates because they are carrying under the thresholds set out in ADR at 1.1.3.6 may display danger signs They may be displayed as placards (250 mm diamonds) or labels (100 mm diamonds) They consider this to be useful to the emergency services.

Ambulances, mobile workshops, engineer's vans and road construction vehicles are examples where hazard diamonds are often displayed.

Some members of the emergency services are concerned that inappropriate action may be taken in the event of an accident involving such a vehicle. They have been advised that the vehicle may be treated as low risk (that is, as if no orange plates were displayed) because the goods should be in limited quantities or under the "small load" limits (see Main Exemptions). If that is not the case, an offence is committed.
Emergency responders will take account of such external warnings in their dynamic risk assessment.

Unless carrying Class 1 or Class 7 goods carriers displaying placards (250 mm diamonds) should be advised to have them removed.  No objection need be taken to the display of labels (100 mm diamonds) as these have no relevance in this context.

Note that vehicles carrying class 1 (explosives) and Class 7 (radio-active substances) will need to display hazard diamonds (at the rear and both sides) as well as orange plates when carrying above the relevant load thresholds (ADR 5.3.1.5).
ADR at 8.1.3 and 5.3.1.1.5 and 5.3.2.1.8 are the relevant paragraphs.

Inspectors should therefore only seek to have orange-coloured panels and placards panels removed or covered when no dangerous goods are being carried on the vehicle. Placards (only relevant for class 1 and class 7 goods)  are often semi-permanently attached to the vehicle using self-adhesive labels. Carriers should have arrangements to remove or cover the hazard diamonds when dangerous substances are not being carried on the vehicle. This would be covered by comment in the UMP or other report form given to the driver.