Even if the Sturgis office had no wall clock, surely the staff (or some of them) wore wrist watches?
KR (the secretary) said she went to the bank at 12.30 and came back 5 minutes later.
When she returned, she said Suzy was on the phone and half-sitting on her desk as if about to leave. After the call Suzy then went to pick up the keys to 37 Shorrolds Road from the key board behind MG's desk, and then took the house details from a drawer. She was carrying her purse and a ring holding the keys of her car, the office and her flat. NH recalled that as she went out of the door she turned back to ask JC 'where did you say my car was?' and then she was gone. This would place the time around 12.40.
So we have NH, JC, KR and even possibly (although we don't know for sure) MG in the office to support this version of the time that Suzy left.
If it was a genuine appointment at SR she would have left at this time, but if it was something she had made up she would have left at 12.40 also as to make it look as if she was heading to SR, just to convince the office staff that she was keeping the 12.45 appointment.
Either way, 12.40 seems logical to me as the time Suzy left the office.