Once I had a setup where you could actually hit and hurt and enemy, it seemed like a good idea to add a quick effect to represent the damage taken. I could also then not count hits that were too close to each other (for fairness).
After playing with a couple of different options, I decided the best way to do this would be to use a particle system (and my other options were terrible). I drew up some basic hit effects, went to add them to the game, and realised that the particle effects would always be below the UI layer (without some plugin hackery).
A quick change to the UI portraits to swap the UI image elements for a couple of Quads that always face up meant that the particle effects could now appear about the characters, which works much better than any other alternatives.
The current result can be seen above. The player character can be seen whacking the enemy characters and also ignores all hits that are too close together on the same individual player.