A thrilling finale folks, let me tell you...
I decided to take my bike so I could hang on until 15:13, then I saw Mrs 3BM's car arrive, I waited she didn't come in, probably held up listening to the end of an Archers, go out, no one in the car, no answer on her phone so she's probably gone up to the school but I can't be 100% sure. Get on bike, cycle up, saw her there so come back just in time for delivery. All OK, in time and nothing broken, yet.