Done 10 of them last week.

Don't post as it needs to be received by them by closing time Monday!!!
You need RF100 signed by a Guard and RF150 to declare.

Easier to explain with an example, My E36 tax is out since 9/11. So I got RF100 signed and stamped by a Guard to state it wasn't used from 1/10/11 until 31/8/13, Then I filled in an RF150 and declared it won't be used from 1/9/13 until 31/8/14. Took both into the tax office and was issued a SORD receipt.