Hartington - many thanks for your thoughts. Yes, it was the same flights, airline, routing etc - just a very different fare. It was not available 24 hours later when I looked! My partner has given up on BA (doesn't like even the chance of disruption and hassle), doesn't like T Cook's baggage arrangements and thinks that Air Canada's 4 flights a day in each direction give her a useful fall-back position so she has actually paid £700 return to fly with them. Thanks to all for their input