Mummyjane -
What Sara has said is right (she knows, trust me). You will be interviewed under caution and then a decision made as to whether you will be prosecuted. IF you admit it in IUC, you may be offered something called an administrative penalty (which is a bit like a caution), but will avoid prosecution. You may have to pay the money back.
As for your "I didn't know" defence - that ain't going to work. Apart from the fact you will have been sent leaflets as Sara says, on every form you have signed for IS or HB and CTB there is a declaration which states that if there is any change in your financial or living circumstances, you must declare it. If you plead "not guilty", the prosecutor will produce everyone of those forms, ask if it is your signature and then read the declaration. THe Court will most likely convict.
On the sentencing guidelines, seeing as it is a year's worth of benefit, the claim was a valid one from the start (ie you didn't start out to commit fraud, it only became fraudulent after you failed to declare your �93,000), you have some personal mitigation I very much doubt that you will go to prison. It is normally cases where the benefit received is in excess of �20k that a custodial is imposed. Plus if you do your best to start paying the money back that will go in your favour - so sell the car and buy a cheaper one (it may release a thousand or so) and show that you are taking real steps to remedy the situation.