hello there, hope you are still breathing there

I think for the first month this is an excellent way to start whilst you get into the swing of things with your allowances. Fear not, any surplus that you save is yours to keep to spend on what you want, that cannot be taken away from you, so alls well there.
You are correct with your overtime, I am with Melanie so I know that they would have explained that to you in full. I think its a good idea to take your money out of the Nat West account, and put it into your co-op account, before you re-set up your DD I am sure come Tuesday this will all be sorted. As far as I am aware you will not be asked for shopping receipts so if cash is the way you feel that you can budget for food and it works for you then i would say go for it.
Good luck with your IVA proposal, I have nothing but praise for Mel and her team, my IVA was approved last week and I know I have made the right decision with my choice of IP, let us know how you get on, and keep posting, there are lots of helpful tips and encouragement on here, I know how scared you must feel but Mel would not put an IVA forward if she didnt think there was a strong possibility of it being accepted. Keep smilling x