As usual… answered my own question just be reading the website…
Both unused miles from your 1000 miles monthly allowance, and unused miles from any packages you purchase will be added to your account as mileage credit. You can use mileage credit on any future subscription months you take out, but please keep in mind that mileage credits will expire one year after they have been applied to your account. All this will be reflected on your account as well as on your Onto app.
If you have mileage credit from previous months, you will see that your monthly subscription starts with more than 1000 miles. If you have purchased any mileage packages, these additional miles are reflected within your total miles available.
You can use mileage credits within the same car tier only. This means if you accumulate mileage with a car that belongs to Car Tier 1 (for example, a Renault Zoe) and you then swap to another tier (for example, a Tesla Model 3), you will not be able to use the mileage credits anymore. If you decide to swap back to the original car tier before your credits expire, then you will have access to use them again.