With the list of cars they offer there should also be the maximum amount of grant they will contribute. If you have this then you should look for cars suitable within this price cap and any amount you could at to it. Then go back to them with the reasons why this is suitable for you.
Current Car
KIA Sportage GT Line Mhev
Previous Cars
Peugeot 5008 Allure Premium
VW Touran SEL
Citreon C4 Picasso