From your reply to Retro89’s question it sounds like it was listed twice on MBs car search, either without or with the sky pack. If so then if the difference between the two APs was £250 then yes, the refund would be correct. If the difference was greater though (and the sunroof was the only difference) I’d be wanting the full difference.
If that wasn’t the case though then the amount you paid for the sky pack as an optional extra should be on your order paperwork and that’s the amount that you should expect to be refunded.