The Royal Mint offer a verification service where they will examine any coins you send in and send it back with a letter explaining their findings and confirming if a coin is an error coin or not. This service costs £20 and it can take a few weeks for your coin to be sent back to you depending on demand for the service. Here is the address:

Dr Kevin Clancy
Director of the Museum
The Royal Mint Museum
Llantrisant
Pontyclun
CF72 8YT