Thanks all - as we are returning via YYZ we will not have to renter the USA so this is not a concern.
However as an aside I can confirm what others say about re entering within 90 days. The last time I was in the US I'm 2009 we flew MAN-PHL-YYZ and then BWI-LHR-MAN. Drove down from Montreal and crossed at St Bernard de Lacolle and we thought we'd have to park up - guy saw my stamp from PHL a week previous and let us proceed I am assuming on the basis of that.