Does anyone know what happened to the Staffordshire Potteries Water Board certificates?

I have in my possession a certificate for the Staffordhire Potteries Water board which according to wikepedia was taken over along with many other water boards by Severn Trent in 1973. I have tried writing to the registrars of Severn Trent hey say they have no knowledge of this. Does anyone know where I can check to see what happened and if this certificate is still valid??