We woke up on Tuesday morning and all we could smell was heating oil. The boiler was leaking.
I didn't have time to deal with it then so I turned everything off and went to work hoping there wouldn't be a massive oil slick when I got home.
When I got back I went about finding the problem, cleaning off all the oil and then letting it leak again. I found the problem straight away, someone in the past had used a hydrolic pipe to from the filter to the boiler instead of the proper braided pipe, this had worn through and oil was starting to weep onto the floor of the boiler.
My dad picked up a new pipe for me the next day and I fitted it and had it working again that night. The pipe cost £5.01 but I'm sure if I got a plumber out it would have been over £100 to have the problem sorted, with call out fees and the like.
I can never understand how people that can't do things for themselves afford to live, that said I'm lucky this was an easy problem and one I could manage.