I would get a plumber in to have a look at your oil line, clock & thermostat. He should be able to get everything to the optimum temperature, time etc, and ensure that you don't have a leak.
Another word of advice is when purchasing heating oil always order 1000l if you can. For smaller drops such as 400l you will be paying a much higher pricer per litre than for a 1000l. This stands to reason as it costs the oil company the same to deliver 400l as it does 1000l. You could also check to see if the company has a monthly payment plan so that you could spread out your costs if necessary.