Not in a huff. Thanks for the advice will go hand luggage only and stand a better chance of making it.
I normally leave plenty of time and was originally on the 22:20 out of JFK but my boss wanted me to change it (he's on same connection) so we can make it to appointment in London earlier! I'm more 'not happy' about the situation of making it too tight a connection in an already busy airport.
We could have had an earlier flight from MIA but sadly it was full, that would have solved everything!