Thanks for your comments. Some food for thought.
I've been down the route of taking BMW finance before to get a discount, but as you say, due to the way interest is front loaded, paying it off immediately erodes any discount.
Regarding at what stage I was at when the dealer was reluctant to offer anything, admittedly it was fairly early - getting the part ex agreed. I will probably be in a position to do something next weekend so will be interesting to see what offer I can get.
Unfortunately drivethedeal no longer appear to do BMW. Anyone know of an alternative site?