Meredith101 correctly states that simply using stolen goods is not an offence. However, it is an offence to 'dishonestly undertake or assist in the retention of stolen goods'.
(Theft Act, 1968, 22(i)):
http://www.lawteacher.net/PDF/TA%201968.pdf
A court might well take the view that your niece has assisted in the retention of the computer through not reporting the matter to the police.
However, a court cannot convict a person of a criminal offence unless the prosecution can prove their case 'beyond reasonable doubt'. To create 'reasonable doubt' she only has to say "Yes, I know that my stepfather got the computer from work but he told me that the company were upgrading their computers and that he'd been offered the chance to buy the computer cheaply".
Unless the stepfather has witnesses who will give evidence to say that they heard him tell your niece that the computer was stolen (or unless she's foolish enough to admit that she knew about the theft), the chances of the CPS even launching a prosecution (yet alone getting a conviction) are virtually zero.
Chris