Mmmmm. I do sympathise slightly.
I had a similar problem with DD when she was a toddler - weaned her on a variety of stuff and she was a great eater. Went on holiday where she got poorly and hte only thing she'd eat while ill was nuggets and chips. So I thought sod it she's poorly, etc it won't hurt for a week and didn't push other stuff.
Then she refused to eat anything else. HV said the same as you JamJamJam - if she's hungry she will eat. So we offered her other stuff, which she would not eat. But we stuck with it and didn't give in. HV said to keep going. She would still drink milk and have weetabix at breakfast (only at breakfast).
She lost so much weight that she became dangerously underweight and we were referred to a dietician at the hospital. Dietician said if she'll only eat chips and nuggets then give her that as we needed to get calories into her. It took about 18 months before she'd eat anything else. Dietician told us to keep offereing other stuff but not to make a big thing about it if she refused as if we did we'd be making food a battle and setting her up for an eating disorder.
But friends and family were horrified thats that all she ate for about 2 years. I wouldn't have let it go on till she was 18 though!