Hi,
I second Bruno's assessment that you are probably into A&P (4th nana) territory, but am not so sure you crossed into Dissolution.
Rod C:
I felt an excited feeling in my diaphragm which suddenly flooded across my whole body in big tingling waves. Wanting to prolong/explore it, I relaxed my legs and then it felt like my body just got dropped a foot or so and more tingling flooded throughout me.
The sensation of falling through the floor sounds like the same way I experienced the entrance to 4th nana, and extreme physical pleasure is also an indicator.
Rod C:
I tried the same thing in meditating the following night and got a similar result but far weaker.
Try not to cling to the idea of experiencing the same thing again, it doesn't often work that way, just watch what happens. It is normal for the entrance to a new state to be impressive, but each time you re-cross that territory towards your cutting edge, the experience will seem weaker.
Rod C:
Since then I have found it hard to focus in meditation (although I have only just started and not really proficient at concentrating).
This could be an indicator that you are into Dissolution, but you don't describe anything that seems to be the A&P event, which is often a big deal first time around, so I think probably not. Try to work on your concentration separately before you begin the practice proper, maybe give counting breaths a go.
See if you can discern the transitions into the early nanas, they may just seem like a change of focus. You may still get a sense of dropping at the entrance to the 4th nana, but look out for the tingling waves of pleasure, watch them arise and pass.
Are you getting a sense of the same tingling sensations during the day, off cushion?
Enjoy it and good luck.