Ordinary Jack by Helen Cresswell
This may have been the cover of our copy (though I have this memory of it being bright red...oh well):
Apparently it's been re-vamped a few times since then:
"Everybody in Jack's family seems to be brilliant - apart from Jack and his downtrodden dog Zero. Even his little sister can beat him at swimming. But Jack's uncle Parker has come up with a plan to make him and Zero shine: they'll pretend that Jack can tell the future! If only they could foresee what chaos the plan will cause . . ."
One comedic incident after another occurs when Jack, feeling depressed that he has no apparent talents as his siblings do, pretends — helped along by his mischief-making Uncle Parker — that he has psychic abilities. I'm warning you now: this one will make you laugh until it hurts! There are so many quirky characters in Ordinary Jack and when they are all in the same place at the same time there's no telling what will happen. This is actually the first in the "Bagthorpe Saga" and I've only read one or two others...must get around to reading some more!
Did anyone else read and love this one?