Change Of Booking

We have booked a flight with BA on 28/10/11. This was to be a connecting flight with Thomas Cook flight later that day. Thomas Cook have told me today that this flight has changed which means we will have to fly ti Gatwick the night before and also have to pay extra cost of overnight stay. B/A stated that to change our flight would cost £146!!! We only paid £77.96!!! We are pensioners and this additional cost is devastating.