I found a product in the supermarket called "Method daily shower spray". It is made from natural ingredients, no chemicals apparently. The point of it is to start with a clean shower screen and spray it on the glass regularly to stop the build up of the scum.

The Ylang Ylang Daily Shower spray is designed to be sprayed on shower and bath surfaces after each shower to prevent buildup and wash away debris and dirt on shower surfaces. The scent is intoxicatingly fresh and beautiful. The spray clings and drips away from shower surfaces taking grime along with it. With regular use, you'll never have to scrub your shower again
It did seem to work, I still needed to do a clean every now and again but no where near as much and without as much elbow grease. (We also used a squeegy on the glass after showers too)