After days of trying to weave my way through thousands and thousands of code, and then trying other solutions (in terms of pre-existing examples that are similar to what I'm already doing) online, I've then found an excellent solution that worked better than mine. It turned out that the game needed a bit more of a 'hands-on' approach, and the keyboard input didn't work out good for the game, so instead, I've changed to a mouse-based input that works by moving the mouse up and down on the tennis court to move the racket to hit the ball to the opponent. I've also made a few changes to the original design, which mostly involved moving the game's logo to the bottom left of the screen, rather than in the middle, to accommodate the 'Round's up' text, which occurs after either the player or the opponent reaches a score of 5. I've also included a clock at the bottom right-hand side of the court, which uses the system's time as external influence.
Also, the colour of the court has been changed to a warmer shade of green to match the astroturf/grass colour that most tennis courts have, and proper court markings have been added, in addition to a backdrop for the scoreboard to increase legibility of the numbers. I've also decided to ditch the graphical movie clips in favour of ActionScript-generated shapes because I wanted the game to be more lightweight, whilst at the same time maintaining the replay value of the original. As for the rest of the detail, I've done improvements to the game's source code to make it more challenging (smaller ball with a higher score limit, but at a slower FPS than the original to avoid it becoming a 'speed run', where the game is finished quickly without even realising the outcome).
In conclusion, the game came out as a result that I've expected from the start, but at the same time, it improved on what my original specification included.
You can play the game here: www.elnormo91.co.uk/Flennis.html
INSTRUCTIONS ON HOW TO PLAY: To move the paddle, move the mouse up or down. Try and get the ball behind the opponent to score.
Enjoy!