When a vehicle carrying dangerous goods breaks down or is involved in an RTC, it may have to be recovered by a towing vehicle. If it is supervised by the emergency services (usually the police), ADR exemption 220.127.116.11 (d) applies.
Otherwise, a new "transport unit" will be formed, and the regulations will apply conventionally. The towing vehicle or low loader (as the case may be) should carry the front orange plate and the driver be appropriately trained (though it would be acceptable for the original driver to travel where that is possible).
If those conditions cannot be met the vehicle should be moved the minimum distance to a safe place pending proper arrangements being made. This could be as directed or supervised by a police officer.
Carriers seeking to arrange recovery of their own vehicles should ensure that the recovery agent has suitably qualified staff to recover vehicles carrying dangerous goods.