Sorry forget to say, it's a flying lap so just start the hot lap and do as many laps as you want.
It took me about 50 laps to get down in to the high 1:21.
Your question has also made me realise the official top gear times are from a standing start so maybe the game is accurate.
I'd guess the at would be around 1:29 if driven by the stig.
As for GT5 keep it in this thread, it would be interesting to compare times.