Ask the company nicely if they can give you a bigger car?
If you do enough business miles, you'll be better off with the larger S-Max if you buy it privately, I reckon it used to cost me about £350 a month all in to run a 10k car inc. fuel, depreciation, servicing and minor repairs etc doing 20K a year.
Buy it at 18 months for half new price, sell 3 years later. Considering you're probably paying £150 a month on a cheap to run golf in tax, you'll be worse off to the tune of £200 a month straight away, but if you can squeeze the maximum 10K a year milage allowance through work miles, that nets you £330 a month in tax free income.
Bearing in mind i'm including fuel in the first figures, that's £130 a month in the kitty for repairing it if something big goes wrong, or all being good it's £130 a month in the kitty to replace it with, which in 3 years would be £4680, a nice chunk toward the next deposit. Bear in mind if you take a car allowance instead of fuel payments, it's not tax free so it's not as efficient a way of doing it.
Having said that, i've just switched to a company car! But I could buy what I wanted, so if you need a certain car you can't have on the company the choice is kind of made for you. I also keep my PAYE below the 40% threshold - I wouldn't be doing it if I was paying 40% tax on it as it would double in cost.