Thanks folks - very helpful. I'll relax! I think out of the last 4 or 5 US flights I've take from MAN via LHR, 2 of them were badly disrupted! One was a same day cancellation of the MAN-LHR leg (and BA helpfully cancelled the transatlantic flight too) and the other was a delay that resulted in a missed connection. Hopefully I've used up my bad luck.