Thanks as always for your insightful replies.
I think you pretty much confirmed what I thought the situation to be.
IN previous experiments I have noticed doing the same or better times at lower tempos while really concentrating on all of the elements. In many of the latter 25's of the sets above I started feeling a little winded.
I think that some peoples body make up and style don't make the higher tempos easy to achieve. I will however try and start at the 1:20 to 1:10 zone as you mention and see what results I can achieve while a little fresher. I think Stuart even once said that based on how he saw me swimming that I should not try and go much faster than a tempo of 1:25. Specially not until I can tune my stroke such that faster tempos give me quicker times.