I've had another look at worked out the problem - the ones that Evotech have supplied have 10mm less thread engagement (using the OEM M12 x 50 bolt) - I literally had three threads engaging on the inside thread of the insert when the OEM bolt is in the new bar end - so I have e-mailed the some pics and will speak in the morning - looks like they have supplied the wrong ones as their stuff is usually spot on.....I never noticed this when fitting them as the thread on the insert doesn't start until a fair bit in.
In summary, with the original bar end there was 36mm thread exposed but with the Evotech one there was only 26mm which resulted in less than 5mm of thread engagement trying to take 66Nm of torque - wish I had paid more attention but as usual I was in a rush before another Skype business call - lesson learned...?..