If its a BMW mini, the Maxi cosi will fit with either with an IsoFix or non Isofix easyfix base. I Spent alot of time researching this!
Have fitted the seat I had in a R53, R56 and R55. So all BMW minis are fine. My friend has the same Maxi Cosi seat in his 53 MCS with the non isofix easyfix base.
But as Rakey says though, get to the shop and you can try them out. You will probably know more than the Mothercare staff if you were anything like me when researching Baby gear!
I reccomend the Quinny Zapp too along with the larger Buzz.