Hi Harry.
Welcome to the world of the Twizy (only 1 Z).
A PowerBox (you need to buy one and make the car what it should have been from the manufacturer - available from Kennith if you search these forums) will tell you the State Of Health (SoH) of your battery which shows how much charge or maximum capacity the battery can take or it’s ‘health’, but expect a 15,000miles 12 plate to be in the 97%+ region and I suppose 30,000 miles to be around 90-95% depending on how the previous owners used the car and battery.
State of Charge (SoC) which is what the dashboard tell you, is how much power of that maximum capacity you have in the battery.
Eg,
100% SoH = 100kw
75% SoH = 75kw maximum battery capacity when charged to 100% SoC
If dash reads 50% charge, the amount of power you have in the battery is 37.5kw (would be 50kw if the battery SoH was 100%).
Range depends significantly on temperature and style of driving. 30 miles is about correct for driving 45-50mph. Normally use the 30/50 rule.
50mph is up to 30miles and 30mph is up to 50miles.
Trying to buy a traction battery is like trying g to buy moon dust. If the battery is owned, then best way to keep it in good condition really is charge to 80% if you’re leaving the car and not using for a few days, then top up to 100% when needed and let the fans and balance charge finish. This can sometimes be hours after the car has ‘finished’ charging.
Most importantly, never trust the mileage guessometer and make sure you’re less than 1mile from a charge point and when the range say 3 miles 
