HMRC is closing down its online computer services for three days starting on Friday 20 November 2009.
Normal service should resume on Monday 23 November.
The shut down is to allow major computer system changes to VAT and Corporation Tax. However, this upgrade has the capacity to affect all of HMRC's services and so you are advised to avoid trying to use them until the Monday.
Useful links:
November changes to VAT online - explanation of what is happening to VAT and how it affects you.
HMRC future downtime schedule - worth reviewing from time to time.