My new employment begins on 22nd march, I am worried about the criminal record check they are going to do!?

I recently had an interview for a customer service position at o2 and have been told that I have the job, during the interview I was also made to fill out a crb check form which will be sent to the criminal records beureu, i have a criminal record whereby I went to jail for 7 months in 2005 for affray and prior to that for 21 months in 1997 for section 18 wounding, they were both violent offences, i am no longer on probation or any kind of rehabilitation as it was quite a while ago, What will my new employer be looking for in the CRB check? And will they be able to refuse my offer of employment if they see the offences I have commited in the past?