It might have been Kellogg's Fruit 'n' Fibre. What I am reasonably certain of is that the jingle waas composed and performed by Jona Lewie of 'You'll always find me in the Kitchen at Parties' and 'Stop the Cavalry' fame.
Actually, Andy, you were right first time - it was Bran Flakes. I was sure that was what had been decided when this was previously askes, and the fact that the jingle says THEY'RE tasty, rather than IT'S tasty would suggest a name in the plural. It's confirmed about halfway down this page:
Ross Kemp did star in the Fruit 'n' Fibre ad though - seem to recall him in a pair of plus fours and other golf gear going on about "apples, hazlenuts, sultanas, raisins, coconuts, bananas".....