Most successful attempt so far. Had a go at 30-20-10 (thanks for the tips Ju-Ju) and set a training program in the Garmin.
Not sure pace was quite right for each bit but I can work on that. How much effort are intervals supposed to be?
Also lesson learned about setting target pace in Garmin. I set the bands far too narrow so it was constantly telling me to speed up or slow down. Think might turn that off for a while.
Generally a success and will be trying again next week 👍