If you're living by yourself then hide a key! It's really quite upsetting when you hear the 'click' of the latch behind you...
Then, buy some fuses (if fused) for the switch box, and work out how to turn the water on and off.
Say hello to the neighbours as well - it'd be weird if you did it in 6 months time (when you want them to do something for you!) but entirely normal if you do it now. I did it when I moved into mine and got help moving all my crap in!
If by yourself make sure you get the 25% discount on council tax as well.
Finally, you've got 6 months (IIRC) to challenge the council tax banding of your flat. A lot of flats have the wrong info - i challenged mine 5months and 20 days after I moved in (cutting it fine!) and got it reduced, and saved several hundred quid. The council had my flat down as a two bed, and had valued it accordingly (in 1991 when the valuation was done). I further amused the council by informing the other occupants of the block of flats of this fact