It was Treets, and as proof of how old the ad. is - it featured a young lady wearing cotton gloves as a fasion item, showing that the chocolate had not melted onto her gloves - how long since anyone under the age of sixty wore gloves in the interest of looking trendy?
It has been used by at least three companies, so most of you are right (except country_boy - it was never Maltesers). Firstly it was used by Treets, then Minstrels and then M&Ms. It could well have been used by others since.
It's NOT Maltesers! They jolly well do melt in your hand. And were Minstrels REALLY called Chocolate Treets? I remember Minstrels as big round things resembling oversized Smarties, and Treets as randomly-shaped nugget-type things.