From the Eastern Airways website:
27 January 2003 | Press Release
Eastern Airways to start Aberdeen-Leeds Bradford on 3 February
Eastern Airways is taking over scheduled services between Aberdeen and Leeds Bradford now effective Monday 3 February, initially mirroring the current timetable of British Airways CitiExpress, which is withdrawing from the route. Flights will operate twice daily, Monday to Friday, with one service on Sunday. All flights will be operated by BAE Jetstream aircraft.
it goes on to say...
At the end of March, Eastern will be taking over a further five regional routes from British Airways CitiExpress, together with 12 BAE Jetstream 41 aircraft. It will also be adding two more daily frequencies on its long established Aberdeen-Norwich route.
Looks like they might be using the 31 on the route initially as they dont get the 41's until end of March