You may not realise you have a diesel leak until you take your vehicle in to a garage and many people confuse a diesel leak with an oil leak. If you notice a puddle forming under your car then smell the substance that has come out. Diesel has quite a distinctive smell and you should be able to tell if it is that. Have a look under your bonnet for areas of puddles where liquid could have been accumulating. Top up your oil and then monitor it for a few days (after driving) to see if it has gone down. Make sure the car is cold when you check the oil levels as oil expands with heat. If the oil has not gone down then you may be leaking a different fluid.
Take your car or van to your local garage and ask then to inspect where the leak is coming from, often it is a simple seal or damaged hose that may need to be replaced which may only cost between £20-£100 for the parts and labour. Diesel leaks should not be left untreated as they can cause damage, make it hard to start the vehicle and be dangerous. Having a fuel leak will mean that your fuel economy will go down.