I hope this helps someone else who has erratic temp spikes when BBT'ing.
Turns out my highs and lows were caused by me taking my temperature at varying times each morning (due to me waking up anywhere between 4am - 6am) but today at my fertility acupuncture the lady taught me that you have to add on or detract 0.5 of a degree for every 30 mins different to your usual wake up time.
So if you wake up 30 mins earlier than usual you add on 0.5 to your temp
If you sleep in 30 mins later than usual then you deduct 0.5 from your temp
I hope I explained that clearly and that it helps someone else!