Hornsea Mere

I remember in years gone by back in the 1970's and 1980's the RSPB had a reserve called Hornsea Mere. I'm not sure where about it is in England or when it left the RSPB's control. Does anyone know why Hornsea Mere stopped being an RSPB reserve, as I have seen that former reserve mentioned on the Internet, but can't find it any where now.

Regards,

Ian.