Looking again at the power adaptor, it is Type C Euro with a 13A adaptor attached. The power supply is from the same manufacturer as other Draytek items I have and is correctly rated for the intended application. The modem was received new, in sealed packaging with the adaptor securely attached to the power supply.
My thanks to Babis3g whose response enabled me to resolve the matter without need for further assistance.
Uninformed and condescending replies are not appreciated.

Uninformed? DrayTek DO NOT ship UK PSUs with Annex B models, so someone has definitely meddled with that product between the factory and you, and that could indeed be a safety issue. Dodgy PSUs can be dangerous - yours might be fine, but if it didn't go through rigorous DrayTek safety testing, there's less chance of that. Condecending? No, prudent safety advice...