Hi Steve
The date of liability is usually either the date the road tax or SORN runs out.
Is the registration document in your name !!??
If not in your name you'll need to send the V5C to DVLA to notify them of the change of keeper. ( They'll not accept a SORN declaration unless you are the registered keeper )
If the V5C is in your name & the previous keeper declared SORN, it may be possible that the reminder was sent to the previous keeper
If you are the registered keeper, you can declare SORN online, via the DVLA website.
Yes, a Sorn declaration only lasts a year & needs to be re-newed on an annual basis.