The thing to do (as I've found) is to get a quote from Lombard Direct/ Lex/ LloydsTSB/ Tesco/ Sainsburys / your bank /whoever at a market competitive rate, print it out, and take it to the dealer and say "match that, or I'm going with them".
This "proves" to the dealer you want the car, and that you are gonna do it on finance - either they do you a deal to meet it (and then get the extra commission for selling you finance) or you will will go elsewhere for the loan...
Also, try to do it towards the end of the month when they need to ensure they hit targets
You need to get a rapport going with the Business Manager (who is responsible for sorting the finance), and not just the Sales guy (who you have hopefully screwed for CD multichanger, mats, and possibly Bluetooth and/or other extras
)... as the Business Manager is the one who will assess the risk and call BMW Finance directly to lower your risk/APR. If s/he likes you...