As nobody has come up with an explanation - I trawled the net and to help any future DIY mechanics here is the reason for my problem:
Obtaining the correct timing belt tension is only possible on an engine at room temperature. Tensioning procedures are always developed for COLD engines, manual and automatic belt tensioners alike. Always COLD! If the timing belt is changed OR adjusted to the recommended spec when the engine is HOT the consequences are likely to be devastating. My error was to continue setting tensioner pointer to align in the square when the engine was HOT. Big no-no. My Gates tensioner was NOT at fault. - I was to blame, although I feel it would be more than helpful if this info was included with the Timing Belt Kit!