With purchases like this I find it useful to make a quick spreadsheet. A column for each type of funding, with a breakdown of all the costs involved per row and a total at the end. To make like easy, try base everything over a set period of time say, the 3-4 years you normally keep a car.
The lease column will be pretty bare as a lot is included already. As for say the 'Buy' column you'll need to include things like servicing and insurance.
Once you have all the numbers entered, it should give you level paying field to compare all the numbers.
Personally I'm not a fan of leasing, as I much prefer to have a asset than to 'rent/lease'. Folk talk about how "convenient" leasing is but convenience costs money and it's not too hard to insure a car and take it to the garage once or twice a year to get it serviced.
Lease companies can also impose daft limits on the milage your allowed to do, before you have to start paying extra! It might only be a few pence per mile but it all adds up!
One other thing to consider, is how much difference MPG can make to your running costs! Not only should a more fuel efficient car better for the environment, it's kinder to your wallet too, both for fuel and potentially road tax.
Miles Per Year ÷ Average MPG x 4.546 to convert gallons to litres. x The cost of fuel per liter.
((12,000 Miles ÷ 35 MPG) x 4.546) x £1.20 = £1870 ((12,000 Miles ÷ 45 MPG) x 4.546) X £1.20 = £1454
45MPG isn't hard for a modern car to achieve and with over £400 saved each year compared to a not uncommon 35MPG. You probably could cover the cost of your insurance each year 'free' by picking up a more economical car.