Thanks for the replies!
dop2000, unfortunately immediate knockback wasn't what I was trying to achieve as it needed to be on bullet collision and not bullet fire, and I needed it to be 360 degree movement, not just left and right. Thanks though!
dieclick, took me a while to get my head around it but you were right! I managed to get a passable effect working by using:
with the blue box being my bullets, which continuously spawn when left click is held and destroy after travelling a certain distance. My player box is 64x64 for scale. I was missing the vector X & Y to actually get off the ground (also found that halving the gravity for the fall made it feel better too).
It's still got some bugs I need to iron out but I'm 90% of the way there. Thanks for all your help!