I agree with Shalimar. The foods you have listed have good aspects and bad aspects, so you need to weigh them up in deciding how often to eat them.
Avocados are an excellent source of healthy fats. However they are very high in fat and calories. Consider them a treat food. Or use a small amount of mashed avocado in lieu of margarine on your toast or sandwiches. That way you're substituting it for something that's higher in fat and calories.
Mayonnaise is usually high in fat. You can get low-fat versions, but they compensate by adding sugar instead. Check out the fat and calorie content next time you pick up a jar. Again, I'd classify it as a treat food. You're better off using a fat-free salad dressing instead.
If you're going to buy cottage cheese, opt for low-fat. It is a lower-calorie choice than most other cheeses.
If you are going to use eggs, consider using only the egg white. The fat and most of the calories in an egg are in the yolk. An egg white omelet is far lower in calories and fat than a normal omelet.
Cashews, like all nuts, contain healthy fats. However, they are still high in calories. If you are going to eat them then keep to small portions. 20 cashews are 199 calories with 13g of fat.
The flavoured tunas can be quite low in fat and calories depending on what they're flavoured with. Steer clear of the ones with olives as they add fat and calories. Just compare the nutrient labels on the tins. The onion and tomato John West Light Tuna Tempter 100g tin only has 81 calories.
Weight-Watchers pre-made dinners are a better option than non-diet pre-made dinners, but check the sodium levels. It would probably be healthier to make the meal yourself from scratch, but these are a handy option for when you don't have time to cook.
Diet cordial and diet soft drinks are an interesting diet conundrum. On the one hand they are a better option than sugary cordials and soft-drinks as they are far lower in calories. However studies done on rats show that rats eating artificial sweeteners end up consuming more calories than rats eating food sweetened with sugar. It's thought that the artificial sweetener confuses the brain. The sweet taste tells the body to prepare for a huge calorie hit. When those calories don't come the body makes you more hungry to try and get those calories from elsewhere. Water is a better alternative, but to be honest I'll have a few diet soft drinks in the evening. Mainly as a healthier substitute for red wine!
If carbs are your major problem you might find a low GI diet would be better for you. If you love carbs a low-carb diet could be too hard for you to stick to. Low GI simply gives you a list of carbs that will keep you fuller for longer. Links: About the low-GI diet
, the Glycemic Index (includes a database of foods)
If you are keen on low-carb you can find out more about it on this site