I got it!
My tempo trainer arrived and I had a chance to use my new toy this afternoon.
Started off at 1.34 (thanks for your advice Ron Bear) but found this tempo really slow. I felt a little unbalanced at this speed so that's something I know I need to work on already. I then played around between this speed and 1.30 for a warm up. SPL at this tempo was 20 or 21 (which is as good as it gets for me at the moment).
I wanted to do a timed 400 and 100 so then played around in the 1.2 -1.3 region. Timed 400m at 1.25 and that felt ok until last 25m went SPL went from 23-24 to 26. I timed my 100 at 1.2 and that felt fast with SPL at 24.
My intention is to practise more at these tempos to get more used to these speeds - would that be a good place a good place to start?
The fastest I tried was 1.16 today and there were certainly a few bubbles generated at that tempo and no chance for a patient lead hand at all.
Thanks for your advice Ron Bear and Terry on where to start. I loved the added focus my new toy generated. For a Friday afternoon swim after a long week's work (and the pressure of the America's Cup in San Fransisco to watch - a great battle between NZ and the US!) I was surprised to read on my watch that I'd swum 2200m and been in the pool for 51 mins! I look forward to playing around some more...