Grand Theft Auto : San Andreas Problem

[mcraig]

I've only just got onto the Catalina string of missions in The Badlands. When I attempt to do the burglary mission where you have to steal the tanker, I can easily get to the yard to sell the tanker, I pull up in the red marker and we go and speak to the man, but when Catalina turns to gesture to the tanker, the man attacks her and she shoots him dead - therefore meaning that I can't finish the mission and it freezes.

Can anybody tell me what the problem is, and how to rectify it?

Thanks.