Hi
thanks for all your advices
ive just been called back from IB+.the agent continued to tell me that 55 min is ok for transit .and I checked on ib website for them the min time to make a transit from T4 to T4S is 30min.
and she told me that the long haul flight may wait for the transit passengers if there are many.if all the direct flights mad-mex operated by ib are sold out they will reroute us to mex via lhr (the ba flight has still 20+ unsold seats).and she said they blocked some seats on these direct flights.and near the departure they will set them free.so we won't stuck at mad for more than one day.and if we want we can move the mad-mex flight to the next in any time before departure without any fee.