Many of us use Livestrong, My Fitness Pal, or CalorieCount as they seem to have the best data and allow for free tracking. The difficulty is with homemade foods and restaurant foods. Most packaged foods are listed on at least one of the above sites. For homemade items, if you eat them often, try creating a "recipe" (at least on Livestrong, that's what it's called) where it calculates the nutritional value of everything in the recipe and in one serving. You can save it for use in the future. It's nice because you can see the effects of adding or subtracting an ingredient on the calorie count, sugar amount, etc (and the effect of changing to a smaller serving size!).
Restaurant food is tough. Some meals are listed in these tracking sites, but most aren't. And in most cases, you have only the vaguest idea of all of the ingredients used.
Also, the analysis is only as good as the data (garbage in = garbage out). So it's important to accurately measure or weigh food, in order to get the best nutritional analysis. (e.g. 1 oz of cashews is a better measurement to enter than a "handful" of cashews).
ETA: I just realized that this is the UK thread; I hope that the websites are universally useful. I know there are a bunch of weetabix listings on Livestrong so that seems like a good sign