In my example above the old website higher price was in line with what the Flight price finder tool was indicating and clicking through that tool also produced the higher fare.

The price offered through the new website was lower than any fare price finder was indicating for the LHR - ARN route on any future date.. SIIghtly surprised at the low fare the new website was offering for this route only 3 weeks out so it could be a fare bug in the new website or BA future bookings are weak on this route