"Use the reference number on your V11 reminder or Registration Certificate to renew your tax from the fifth day of the month in which the current tax disc expires."
Which to me says you can renew from the 5th of the month in which it expires
Or: "You can renew your tax up to two calendar months in advance in person or by post at a Post Office® branch that deals with advance applications or a Driver and Vehicle Licensing Agency (DVLA) local office."
All taken from the link below
http://www.direct.gov...ourVehicle/DG_4022052