I decided you were correct Gray and so I started by searching Sheffield United Ltd at Companies House. The details disclosed that the register of members had not been available for public scrutiny on the site since 2016.
In view of this I called Capita who are The company’s registrars. Having taken my details they confirmed that they had me registered as a shareholder but that they would never disclose these details to anyone other than myself. They could not advise how anyone might have obtained my details but suggested that I called the FCA.
This I duly did but was unable to give the name of the organisation that called me from Washington DC so their advice was inevitably restricted. They confirmed that only an authorised broker could trade in the shares but they could not check this without a name. Nor could they say whether it was a scam but in their experience if someone was cold calling to buy shares it was quite possible that it was a scam (possibly a boiler scheme).
So roll on GDPR and meanwhile if you receive a call from a nice gentleman in Washington DC you have only called in to put a shelf up for the person they are calling who by coincidence is away on business for a few days!