I would cut out or if you can't do that then cut down the sunflower seeds, they are very high in fat and can and do cause lots of problems in parrots when fed in high amounts, also to many peanuts are not good either.
Is Jasper the grey? If so then jasper does not need grit, grit really should only be given to birds that can't husk their seeds.
You also said they get veg stew, now their is nothing wrong with that, depending on how it's cooked and what's put into it, e.g. Salt etc.
You also say corn beef, have you had a look and read if the info on what's actually in the corn beef?
Just by reading Jasper's diet I would say it's probably a dietary problem, obviously I'm no avian vet but I would suggest you take jasper to an avian vet and get full blood work done, with a diet like that which contains highly fatty foods, salt maybe too, there's a chance of some liver problems and vitamin deficiency, if that's the case then these things can be sorted with the right diet, vitamins etc.
Again this is just my personal opinion as I'm no vet.
Have a read if the diet section as there's some fab info there.
Julie