BA will send you a briefing on the simulator assessment. I was sent one which included information on how to fly both the B747 and B757, in the event of the B747 sim being tech.
At a recent assessment on the B747 we were told they are going to start doing the assessments on the B757 only, I think from November. Be patient for BA to send you this information which they did for me 3/4 weeks ahead of time.