I live in a hard water area so shower head regularly gets a build up of limescale on the jets. I clean them with an old toothbrush but this does not remove it all. Have been advised to use either calgon, white vinegar, lemon juice or cheap cola.
I live in a hard water area too. I bought some Oust at the market today. I`m going to stick the shower head in some of the solution. I saw one of those cleaning women on the tv once descaling a tap. She got some cotton wool and soaked it in white vinegar then wrapped it around. I`ll probably do that for the shower head tubing and use the Oust for the head.
I always use vinegar to descale everything, it's cheap and there are no nasty chemicals in it. Take the shower head off and soak in vinegar for about 30 mins, by the way, I use ordinary malt vinegar don't know if there are any differences between them.
I know it may sound tedious but clean the holes individually with a toothpick which will remove all the little black bits and scrub after with an abrasive sponge and fairy liquid and if necessary a little bleach.
I use Jebsol 51 descaler. It's very strong and does a great job. Haven't seen it for sale in any shops though, I buy it in a 5 litre container from a local chemical company.