What you’re experiencing @Username06 is the fact that not all calories are the same!
Calories from carbs will have more impact on your weight than calories from protein or fat, because carbs instigate the release of insulin, and it’s insulin that results in the laying down of bodily fat.
breakfast is either fruit/yogurt or weetabix/porridge usually with a measured amount of jam or peanut butter.
The cereal is going to be high carb, especially if you’re having it with jam (peanut butters vary but some can be very high in carbs), so you’d be better off sticking with the yoghurt. Fruit can be high in carbs, depending what type of fruit you’re eating - berries are generally the best to choose. If you don’t want yoghurt every day, then how about eggs? High protein and also good fat.
Lunch can be anything from a tuna/egg salad sandwich (made at home)
a salad, cheese and crackers, sometimes a mugshot if I'm strapped for time etc with fruit or jelly.
Again, potentially a lot of carbs here - especially the mugshot. And also highly artificial. Could you just go for a salad - with a protein element and also an oily dressing? You can prep stuff in advance, e.g. hard-boiled eggs, and just have stuff to grab out of the fridge, so it really shouldn’t take long to put together.
Dinners for example this week I've had tuna pasta salad, pork stir fry, chicken, veg & rice, omelette & the odd ready meal on days my DD has clubs and I have a busy day etc.
More carbs - the ready meals may or may not be carby, but generally (especially as most include rice/pasta/potatoes), chances are they are also carby. Choose other vegetables with your meals wherever possible.
I do have a small sweet treat in the evenings
More carbs. Evening snacking is a habit (which you’ve referenced/acknowledged), and chances are you’re not really hungry. But if you are still hungry, can you replace with cheese, or olives, or something like celery and cream cheese or pâté?
Given that you’re counting all your calories, and it’s not working for you, I’d suggest that you look at ways of reducing the carbs within your calorie count. Focus on adding protein (natural protein, not protein bars!) and also fat - avoid low fat/lite foods - cook with butter and olive oil. You’ll feel fuller for longer.