Hi madmanx,
I can certainly help you with this, as the same thing happened with me. You get a year to spend the backdated DWP payment before it's classified as savings income. After that, then it will affect your UC. I didn't declare mine to UC for a couple of reasons: 1. I didn't know that I only had a year to use it, and ended up locking it away in an ISA, for which I am now being penalised via my UC and 2. When UC asked to check my bank statements, they went back first 4 months, then seen that I had over £6,000, and told me I had to give them a years worth of bank statements, including all my ISA account details. Cut the long story short, they are now taking it back off me as an overpayment on my UC, I also got fined £50 for not telling them as they said that basically not knowing isn't an excuse for me not informing them.
Hope this information helps.
Regards, Chris.
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
