With
s = principal
n = no. periods
m = periodic payment
r = periodic rate
b = balloon
where the balloon is paid at the same time as the final payment in month n
![eq](https://cdn.statically.io/img/latex.codecogs.com/gif.latex?s%3D%5Csum_%7Bk%3D1%7D%5E%7Bn%7D%5Cfrac%7Bm%7D%7B%281+r%29%5E%7Bk%7D%7D+%5Cfrac%7Bb%7D%7B%281+r%29%5E%7Bn%7D%7D)
The present value of the principal is equated to the net present values of the payments; then the summation is converted to a closed-form expression by induction.
∴ s = (m - m (1 + r)^-n)/r + b/(1 + r)^n
∴ m = (r ((1 + r)^n s - b))/((1 + r)^n - 1)
Assuming the dealer contribution is deducted from the initial amount.
s = 20000 - 2000 - 1000
b = 1000
r = (1 + 7/100)^(1/12) - 1
n = 36
∴ m = (r ((1 + r)^n s - b))/((1 + r)^n - 1) = 498.12
or calculating with APR as a nominal rate compounded monthly
s = 20000 - 2000 - 1000
b = 1000
r = 7/100/12
n = 36
∴ m = (r ((1 + r)^n s - b))/((1 + r)^n - 1) = 499.87
Looks like the website is using nominal rates. However, UK uses effective rates.