I see the problem, there was 2 properties, a showroom with 9 outdoor parking spaces, and the second, a petrol retail station, you have taken both and you are either getting 2 separate Bill's or the council in there wisdom have combined the 2, without reviewing the petrol station!
There will still be rates to pay for the additional parking, the cost of which you will have to look into personally, I would look at similar sales pitches on voa importantly in your local authority area to get a idea of cost.
My advice, listen carefully the showroom and 9 outdoor spaces need to be totally separate from the expetrol station that is now car sales parking in regards to billing, I would put the expetrol station in your business partners name and the showroom in yours, or whatever .
If the 2 were combined, which afteralls is less paperwork for your local authority this could possibly result in a bigger overall bill than having ex petrol station separately rated, it maybe actually under the 15k discount (look up rates discount exemption) and result in paying no rates whatsoever, so, as the actual properties were rated separately before I can see no reason that they cannot again (I am basing this assumption on they have billed you as one).
So, rather than worrying about paying or getting your grant reduced by £2800 I may have just saved you £10,000 +...........
To be honest, and I am not knocking you, but for you and anybody else reading this, ALWAYS check all figures before jumping in feet first, a good example, again, my friend has just moved from a pitch to a indoor warehouse, fantastic spot secure, clean, but guess what, his insurance company have now set up many hurdles for him to jump before they will quote as it is allegedly in a bad area !
I wish you well.