Where could I find my Driving License number ( except the obvious!)

[escape]

Good Morning!
I have a Police speeding fine letter to return TODAY, and I cannot find my drivers license ANYWHERE to fill the number in! Is there anywhere I may retrieve the number that you know of? If I call the DVLA maybe?
Would it be possible to return the letter without the number and a note - as the m receiving the letter within the time limit is the most important part, not shirking anything here..
All advice gratefullly received!

If you phone dvla they will tell it you.
Tut tut at speeding lol

Thank you!
And I know, slap wrist - still don't know where the street is, but 47 in a 40mph zone, appar..