I have said this before, but for all the newbies to the challenge;

For the first month I took a guess at the budget but I really had no idea how much I was spending.

I found the best thing is to have a guess the first month but SAVE ALL OF YOUR RECEIPTS (and yes, that does include the ones from the local shop when you run out of milk ), then at the end of the month add them all up and have a look if you were on budget, if not, it doesn't matter because your budget was a guess. For the next month you can use the total of your receipts and take a bit off to get a realistic budget, just shave off as much as you think you can, it could be £1 a week or £20 a week.

If you are currently spending £200 a month don't suddenly drop it to £100 - you'll never do it but if you cut £20 a month off for a few months you'll get there eventually without feeling you're missing out on anything.

You might think you've done everything you can in the first month but someone always comes up with a new idea and, if you try it you often find you save a bit more (unless you are SarahSaver who is an expert at this and could only save more if she turned her back garden into a paddy field and lived on rice ) In the first month I thought, I'm planning meals, I couldn't possibly save any more, but then I started double cooking and freezing, and making soup instead of buying it in tins.

I've been doing this since Feb but only saved ALL of my receipts last month, it has given me a much better idea of where it is all going. Budget for next month is £80 for 2 of us, I'll start on 30/4. I could probably save more, by eating different foods but I can afford to live on £80, I have saved a lot of money since Feb. and I don't waste food or money any more. So I'm happy with my budget.
By request, here is Spiggles wonderful helpful post