Ok so water and concentrate sulphuric acid must add up to 100cm3, l must add 5cm3 of acid and about 50 -70 cm³ of water?

However, you don't add a fixed amount of water like "50-70 cm³". You add the 5 cm³ of concentrated acid to the volumetric flask, and then you add enough water to bring the total volume up to the 100 cm³ mark. The exact volume of water added will be approximately 95 cm³ (100 cm³ - 5 cm³), but you don't measure this water volume separately. You simply fill the flask to the mark.