Ok, so let's break down the running costs.
First, the assumptions made (people in the know please do correct me):
Cost of clutch on BMW: £500
Cost of Tax on Fiesta: £60 p/a
You cover 14k a year, or 1166 miles a month on average, and that the Fiesta does 60mpg, the BMW 30mpg.
Not taking into account other running costs such as servicing & MOT, monthly costs:
Fiesta Loan/Finance: £330
Fiesta Fuel: £119.10
Fiesta Tax: £5
Fiesta Insurance: £100
Total: £554.10 a month
£6648 a year
Now the BMW:
Fuel: £229
Insurance: £117
Tax: £20
Total: £366 a month
£4392 a year
So you're £2256 better off after just one year by keeping the BMW. Even after replacing the clutch you're still massively better off.
Personally, I'd quite like to drive one of BMW's finest cars made and save a wad of cash vs a new Fiesta.