I would expect that as you signed a contract for a certain figure they'd need something in the terms and conditions to enable them to change it and a timeframe in which they can - seven weeks seems a lengthy time to find a mistake. The best bet may be to talk to your local trading standards team.
If they try to force the issue you are within your rights to cancel the order and get any deposit back on the grounds that it's their mistake and that, effectively, you were misled.
In the meantime I'd also look on the BMW website and configure the car again for the options you'd specified and try to find the difference. Also check What Car's website for an approximate valuation on any vehicle you may be trading in.