Jules, a cholesterol of 7.8 is high. As someone has already said, the accepted 'normal' upper limits are between 5 & 5.5 (it depends on the lab doing the analysis). As important as total cholesterol is the ratio of LDL (low density lipoprotein) to HDL (high density . . .). Somewhat counter-intuitively, LDL is 'bad' as it circulates in your blood in big floppy bags of fat that can contribute to atheroma (the fatty streaks of damage to blood vessel walls that are the cause of heart attacks, some strokes and other vascular disease). HDL on the other hand 'mops up' spare cholesterol and returns it to the liver.
Diet is important, but I was taught that it only contributes perhaps 10 - 20% of your circulating (ie in the blood) cholesterol. The chemical pathways that make cholesterol in your body (it is an important constituent of cell walls and some hormones) make the rest, and in some people they just work better. The cholesterol-lowering drugs, statins, mentioned by another correspondent, work by inhibiting another chemical which allows a step in this cholesterol-producing chemical pathway to happen. They work very well, but as with all drugs, have side effects which need to be considered when weighing up one's approach to cholesterol.
In summary, it's not as straightforward as it would seem. Cholesterol is important in the body, but high levels are associated with increased risks of cardiovascular disease (heart attacks etc). You must always consider other risk factors for cardiovascular disease; smoking, high blood pressure, & being male (to name but a few), are more important. Diet can make a difference, but some of us unfortunately just produce more cholesterol.