decent as in £22k starting salary and a free mini.
No, no, no...
A company car will cane your wages with tax, might as well earn 18k for what you'll take home, plus how often would you get to use the car in London? And when they say "free", if you get to use it for personal use and they pay the fuel then that's even more tax you pay.
Take it from personal experience, I left a 30K+ a year job with a car for a job with the same salary and no car and was £300 a month better off.
I'm not an expert on this, but the killer is free private fuel, which could be more than the assessed benefit of the car itself.
If you weren't given free private fuel (a fixed charge, no matter how much you use) I reckon the taxable benefit would be about £2800 (without knowing cost of car exactly). At that salary level 22% tax would give a charge of £616 or about £52 per month out of your take home pay. - I'd say that's a good deal as maintenance and insurance would probably cost that much for you. The free fuel would more than double that liability and would only be worth it if you were going back to see you girlfriend very regularly !
Like I say, don't take this as gospel,do your own research on the HMRC website, but it's worth considering.