Most of this is gleamed from the HTC Desire thread probably not as good a deal but a deal anyway.
I went through this process and while getting the HTC Desire I asked if there were any options to buy a Blackberry - was interested in the 8900 for the wife but they didn't have any in stock only the 9700 at £325.
Checked around to see comparable prices and indeed looks like a good price.
Especially as O2 are doing Simplicity 30day £15, 300 mins, unlimited txt and Blackberry Unlimited. save £5 if you do 12 month.
So follow the tried and tested formula below.
Edited to include credit to speedski:
"Phone 202 - press option 2, option 2 and then finally the third option 2 for the 'if you're thinking of leaving o2'
DO NOT CALL SALES OR UPGRADES - YOU MUST CALL THROUGH TO 202, opt 2, opt 2 and opt 2."
"When you get through say you would like to buy a Blackberry 9700 outright for £325 - do not mention your renewal, don't mention upgrades."