I was going to say similar to UpStartCrow on P3.
I think I'd have 2 problems with envelopes of cash - one being if you were ever broken in to, then that's all - all your cash is gone and you would be in trouble. The other being that if you have the notes there readily available, then it's too easy to 'just' take it as it's handy, for something you want, I'd have thought.
The key is, to also remember to put in your budget all the things you pay annually, as well as the monthly things - be that TV Licence, MOT, various insurances, Christmas, Birthday presents, holidays, and remember things like Water Rates and Council Tax which are paid in some months and not others.
Then add in the inevitable repairs pot (or envelope if you go that way) - cars, boiler, white goods, etc.
You say you've written down what you spend. Have you included all the "little" cash purchase you make now - be that lunch on the way to work, coffees out, magazines, parking fees, or whatever ? It is surprising how they can add up.