Diesel and petrol have swapped places in the public perception of "cheaper" a few times over the past decade, and the honest answer is that neither is simply cheaper — it depends on the pump price at the time, how many miles you drive, and what kind of driving you do.
Pump price
Diesel and petrol have historically traded places on which is pricier per litre, depending on wholesale market conditions and seasonal demand (diesel demand tends to rise in winter due to heating oil competing for the same refining capacity in some regions). Check the national averages or your own county page on this site for the current gap where you are, since it isn't fixed.
Fuel economy
Diesel engines are generally more fuel-efficient than equivalent-sized petrol engines, often returning 15–20% better fuel economy (miles per gallon) for similar performance. This is the main reason diesel has traditionally suited higher-mileage drivers: even when diesel costs slightly more per litre, using less of it per mile can still work out cheaper overall once you're covering enough miles.
Where the mileage breakeven usually sits
As a rough rule of thumb, diesel tends to work out cheaper overall for drivers covering somewhere in the range of 10,000–12,000 miles a year or more, largely motorway or longer-distance driving, where its fuel economy advantage has more distance to make up any difference in pump price or vehicle cost. Below that, especially for shorter, stop-start urban journeys where diesel's efficiency advantage shrinks and diesel-specific components (like particulate filters) can face more wear, petrol is usually the more straightforward and often cheaper overall choice. This is a general guide, not a fixed number — your actual breakeven depends on the specific vehicles being compared and current fuel prices.
Other running costs worth factoring in
Diesel vehicles are often slightly more expensive to buy new or service, and low-mileage urban diesel driving can lead to particulate filter problems over time. Petrol vehicles (and increasingly hybrids) tend to have lower upfront and maintenance costs, which can offset a fuel economy disadvantage for lower-mileage drivers. There's no single right answer — it's worth weighing your actual annual mileage and driving style against current fuel prices rather than assuming either fuel is automatically cheaper.