Soak Shower head for 1hr in vinegar then rinse and replace.
If you don't want to remove your shower heads--fill a sandwich size bag (or larger depending on the size of your shower heads) with vinegar and secure them around the shower head using clothespins or rubber bands. Let soak for a few hours and then remove. Use an old stiff tooth brush to scrub away any loose scale. Rinse.
Try using Barkeepers Friend! It has done wonders for us when it comes to limescale.
If the vinegar in baggie doesn't work use the baggie with CLR
Gk and Lynn both advised you right. Let the vinegar soak for at least a few hours, maybe even overnight. It could take a couple of tries if it is really caked on, scrub with a scrub brush or toothbrush to remove what is flaking off before you repeat the process. CLR is also a good choice, but will need gloves onto work with it, especially if you have sensitive skin. Vinegar is also cheaper. Good luck, Jan, you can clean the shower head!
Thanks for all your ideas. I’m trying the vinegar method. Sadly barkeepers friend not available in the UK!