How to obtain my UTR number?

I have to fill a Self-Assessment and Tax Return for 2006/07. The only paper I have is P60 and it does not have a UTR.
Please, advice should I have to wait fro a letter from my local Tax office or I have to register first somewhere and then wait for the letter with my UTR (I would like to fill in the form on-line).

Cheers.

If you fill in by hand and use the post (how old fashioned!), you can leave off the UTR and they will add it when they process your return.

Otherwise, you'll have to call the HMRC and arrange to have one assigned to you.  This could take weeks or months.