Hi Lucy,
I am not a long time owner and expert, we have had Herbie for 9 months now and it is his first birthday this month. But I hope what I have learnt might be of some help, we have fed Herbie a number of different foods, he loves toast and porridge in the morning and pretty much anything else he can get his beak into the rest of the time.
It might be coincidence but we had some health issues with him recently, he picked up a bacterial infection from somewhere and along with antibiotics the vet recommended Harrisons pellet food (you can find this online from many pet retailers). He has since got better (down to the medicine I'm sure), but since being on the pellets he seems much more calm and more affectionate than ever. We still also feed him fresh veggies and fruit. My girlfriend makes a lot of purees for him, made from vegetables and fruit and often hand feeds him from a tea spoon. Our vet told us that hand feeding can help continue the bonding process and even though your bird is much older than Herbie it might help bonding (once he is comfortable to have your hand near him).