For the last few years my tax calculation has been very close to the final outcome for the year

However last year my tax code was adjusted by a VAST amount
I requested it be revised three times to something more realistic, but it was not

As I suspected I had not paid enough tax last year, and so knew I would receive a tax payment request
This arrived last week, but with no details of how to make this payment
This morning the payment calculation arrived :
Total to pay £xxxx.xx - total to be paid - £0.00

As I knew I was going to have to pay this tax, due to their failure to correct my tax code, I invested this money ready for this payment to be made

I therefore phoned them...
Yes, I am due to pay tax, but nothing is due
What they are going to do is adjust NEXT years tax code, ie 2011/2012
WHAT?

Has anyone else had this?

In effect this means I can sit on this money, make about 8% interest for at least 18 months

In the past my tax code has never been changed and last year was the first time this happened, never again!