The Goldsmiths’ Company Assay Office is where hallmarking began. 

Quality of mark

The leopard’s head mark of London has been used by some of the finest craftsman in history and continues to be the mark of quality for many of the leading jewellers and silversmiths today.
The Assay office have been testing and hallmarking precious metals for over 700 years.
The Full Traditional Hallmark comprises five marks:

Sponsor's mark - compulsory
Traditional fineness symbol
Millesimal fineness mark - compulsory
Assay Office mark - compulsory
Date letter mark

For further information about Hallmarking, click on the link below

assayofficelondon

The London Assay office Hallmark information guiding you through each mark and what they tell you.
Your guarantee that the item you have purchased has been tested to confirm the metal is as the seller has stated. Look for the Hallmark.