Being diabetic doesn't mean that you've got to go down the route of Atkins style diet this isn't always needed.. In your mum's case it is matching her insulin intake and carbs to obtain good target ranges that your mum is comfortable..
As your mum is on Novomix, which is based on a split of quick acting and mid acting (around 12ish hours) insulin requires matching, as iHs as said, 3 main meals and 3 further snacks, mid morning, mid afteroon and supper..
To get Novomix to work properly, then one needs to be eating the same amount of carbs for each meal/ snack and attempt to keep very similar timing for each meal/snack
i.e, NOTE times, carb amounts need to be sorted with HCP and your mum to suit her requirements these are for example purpose only
Breakfast 8am 40g of carbs
mid morning snack 10.30am 20g of carbs
lunch 1pm 30g of carbs
mid afternoon snack 3pm 20g of carbs
Tea 6pm 40g of carbs
supper 9pm 20g of carbs
As what to eat for any particular meal/snack...
Mum can have her spud+meat and veg if she wants, all she needs to remember is that items such as the potatoes needs to fall within her carb allowance for that meal, if the potato portion size makes her meal feel small, then add extra meat and vegatables such as cauliflower that are very low carb count (meat is zero carbs)
Suggestion for food, fruit (if mum doesn't feel like a snack, then she can swap her allowance for somthing such as orange juice, apple juice etc)
Crackers + cheese
rich tea biscutes aren't high in carbs, digestives are just under 10g of carbs..
For a treat, you be surprised how low carb something like a chocolate eclair is, even cream horns aren't two bad either
Nuts are quite low carb (check out each type)
Then you've got your salad options
Even a packet of crips only have just over 10g of carbs in them, another little treat to make life interesting..
If mum prefers to have a sandwich, then have a look at an wholemeal type bread, Garnery is a nice one or some of the low carb/ weight watchers alternitives that can be found in the super market..
For puddings, you've got sugar free jelly, even angle delight (which you would have to count the milk content) Yogerts can be found in low in carb content.. You can make a lovely low carb triffle, even custard can be made with sweetner instead of sugar to lower it's impact..
and if she fancies a bit of ice-cream, then perhaps invest in an ice-cream maker you can make ice-cream using sweetner instead of sugar, add some berries, such as fresh strewberries etc, nice treat..
In the main if mum can be regimented in her routine, she will fair a lot better in the long run, as she and her diabetic team can talyor her insulin needs to suit, and hopefully with not to many alterations to her diet..