Thanks guys - it's good to know that I'm not the only one in this situation and that other people have found the timing quite tricky too.

I've told DP that we need to get ourselves organised and diligently got out the thermometer, bought a special notepad that has graph paper and marked out the axes.

DP was crashing around at 5.30am this morning so I thought, "bugger it, it will take me ages to get back to sleep so might as well take my temperature" ... ONLY TO FIND THAT THE BLINKING BATTERY IN THE THERMOMETER HAD GONE.

Talk about disappointed!