Since I put Digby on a diet the variety of things he eats has actually improved. What I've done was to stop giving anything else during the day apart from the pellets and pellet bread.
His schedule is at 7am and 7-30pm he gets mash (veg plus pulses) and a tablespoon of sprouts, sprinkled with ground up linseed (for Omega 3) and sesame (good calcium source).
So for 12.5 hours he only has pellets available and he eats them now really well. When we eat our dinner I cook a slice of carrot, a floret of broccoli, sometimes some french beans too for Digby. He eats all of them. He never ate broccoli before but he does now! He ate a whole floret today (apart from the green bits, he seems to prefer the stalk!). He is a lot more keen on cooked carrot too (cooked carrot is better than raw for vitamin A)
I've cut down on fruit during the day too. he gets half a grape but I need him to really want it and to eat it fully because it is his medicine carrier. On top of that i tried him the other day with Kiwi and he loved it even though he never wanted to eat it before no matter how many times I offered.

He also now loves blueberries but wasn't bothered before.
So the new foods he eats:
Broccoli
Kiwi
blueberries
leafy greens at times
microgreens
and eagerly eats pellets
I am happy

just to clarify - I didn't starve him for 12 hours when I wasn't sure whether he would eat pellets well or not - I just gave him bird bread made of pellets as I knew he would eat it and offer him some pellets too during the day. And now I am cutting down on the pellet bread and he eats more pellets instead.