Can I Just Use Spa Sanitiser?

Is it ok just to use a sanitiser (Spa Chlorine or similar) in my spa?

NO. It is extremely important to maintain the water balance on a regular basis. A combination of the pH level, Alkalinity level and Hardness level dictates whether the spa is corrosive, scale forming or balanced. If these levels are mot maintained the water can corrode the spa equipment, particularly the element, leading to an expensive repair bill. The water will also not look and feel good.