I live in Norfolk where we have particularly hard water. This means that limescale can very quickly build up on the shower head. Luckily, it is very easy to clean and prevent.
Bathroom Cleaning Spray and Scented Vinegar will both do the job nicely. I personally use Scented Vinegar (in fact, I use it for most cleaning tasks) but since you likely already have Bathroom Cleaner in the room, it makes for an ideal substitute.
Start with the shower running and use a wet Copper Cleaning Pad to give the head a good rubdown. Turn it off and wipe dry (I use a Linen Cloth). Now spray on your Cleaning Spray or Scented Vinegar.
It should only need a few seconds before you wipe it dry again.