Just got these figures from the finance configurator...
For a bog standard TA Cross, over 36 months, with a £3250 contribution from Fiat via the IAM plus £2700 deposit from me, £4300 final payment, and 35x £170.87 per month, the total interest payable is ... £0.45. Winning!!!
It wouldn't let me put any more in as a deposit - the max it would accept (including the £1850 which is the other bit of the Fiat allowance) is £4550. I assume that the maximum deposit varies depending on whether you spec it up, which presumably is why Bundes was able to have a higher deposit of £3490 + £3250?