120 miles a day = 2 hours driving x 5 = 10 per week x52 = 520 per year / 12 = 43 hours per month giving a grand total of = 21 1/2 days a year in the car commuting to work
Each journey costs you £35 a day roughly.
40mpg costs you £18 a day roughly
£17 per day saving x 5 = £85 per week x 52 = £4420 per year / 12 = £368 per month saved over the zed.
So it's either 20 days a year miserable and mundane with an extra 4.5k in your sky rocket or or 20 days inspired and happy but in the long run back to miserable again because you know that 4.5k of your pay rise has just gone up in smoke.
Alternatives to my mind are super, super uber cheap derv to knock around in (500 quid plus running costs) or sling the zed