Walking a mile or so during week when at work
That's about 3000-4000 steps. It would indicate a very sedatory life. For comparison, I've usually done 5000 steps by 10am every day. I generally do around 20,000(+/-5000) steps a day.
Not doing much exercise, while not healthy, won't much affect your weight loss in any meaningful way. But what it does affect is how many calories your body needs.
You don't mention your height, but you will need to eat significantly fewer calories than someone else the same size as you, if you don't exercise. Plus if you are looking to lose weight, you need a calorie deficit on already low calorie needs.
I suspect you are used to consuming far more calories than you need (so put weight on). You need to re-teach yourself not only that you need fewer calories day-to-day, but that you need even fewer than that to lose weight.
You need to get your calorie intake way down. Do that my thinking about what you eat so that every calorie is as filling as possible.
I have been using the app but haven't found it much help
In what way is logging your calories mutual helpful?
Have you tried weighing out the food you eat to check your portion size?
Are you making these evening meals you mention from scratch? Or eating ready meals?
What is a typical daily calorie count?
Do you look for low fat meats? High protein yoghurt?
How many vegetable portions do you have per day? Is there any reason you can't have vegetables at lunchtime? They are more filling and less sugar than fruit and sweetened yoghurt.