www.google.com
just stick in what you hear ie GBL46M and you'll usually get exactly what you are looking for, I've just done GBL46M and now know its flight number is BA6946 from Gatwick to Malaga, can see there's a full flight plan available to view, know that on one day A320 G-TTOE flew it and was even offered a link to track the flight's active progress. I'd imagine that you can do this for many callsigns/flight numbers. Its a good starting point anyhow!