For your convenience, use 4 or 5 level scoops to measure 40 or 50g Purition, you can buy one here. They're £4.99 and ship for free when purchased with any 500g bag or more.
These are available for your convenience only (we're not making money from selling these). We'd always recommend using something you have already.... e.g. 4 heaped teaspoons (the ones you stir your cuppa' with) is roughly 40g Purition.