Your budget may be better than you think - it depends how your company operates its car scheme(s).
Are they saying you can have £7000 p.a. + diesel at cost to run your own car? Or is the fuel paid on a mileage basis?
If the former, that equals about £12,000 p.a. assuming (say) 50,000 business miles at £1.00 per gallon based on 50 m.p.g. The tax-free rates approved by HM Revenue & Customs are 40p for the first 10,000 miles and 25p thereafter. That equals £14,000 p.a. tax free. You can claim the difference of £2000 from HMRC as a tax deduction at your marginal rate(s) of Income Tax.
That should give you a MINIMUM monthly budget of just ~ £1050.00 per MONTH to fund everything - which leaves ~ £600 per month after the diesel costs. Even more, if the company are more generous with mileage alloewances.
If it were me, I would be looking for a low mileage 18 month old diesel and change it every year while still in warranty.