it is the salary sacrafice option then - just be clear on what it means for her pension as of the 1st April 2015, as this will impact on this.
The figures quoted take account of her mileage payments for doing 6kpa business miles, to get a fair comparison she needs to zero this down, this will also help if she goes over 10kpa as they remove business miles from the overall mileage and so long as she hasn't gone over on 'private mileage' then they won't charge her extra (ie if she signs up to 30k private miles and does 45k of which 15k are business she won't be charged a fee when the car is returned).
the comment at the beginning about private mileage is just confirming she is not being paid for her own personal use (which would be an additional taxable benefit), this is what you'd need it to say, so that's ok.
All fuel does come out of her pocket but she can claim the mileage - this goes back to the key point in both of my posts though, her quotes already take this into account hence the low lease figures you are being quoted which look really attractive - net down the business miles to zero for a fair comparison. The £125pcm takes account of her 500 business miles a month, which is why it looks attractive. If she got a private lease/PCP/ 20 year old Fiesta then she can claim mileage.
If she's doing 10kpa and lots of stop start a diesel makes no sense at all - more expensive to rent and tax, cheaper to fuel (slightly) but it's worth checking her employers mileage rates - for example, she may get 30p per mile for a 1.2 but a 1.6 diesel would get 40p per mile; this is true of a lease car through Knowles or anything else.
It's easy, but it is not good value - my management accountant set it up at our place and she's just replaced her old car with...a privately bought one. 'Nuff said...
many moons ago I had a Punto 1.9JTD; I got 45p per mile even though it did 55mpg and I did 8kpa for my employer - it made me loads of cash, I suspect your wife could choose a used car and be no worse off. 6kpa with an essential user allowance even at 25p per mile is £1,500 in mileage and £600-ish after tax; £170 per month over three years with your part ex can buy a decent car and leave you with something that has a value in three years time...or you could pay £1,500 a year over her mileage etc. for an A1 then just wave it goodbye after 3 years!