A few days later this is how things have gone.

Day 18 LH surge detected with OPK and DTD late that night
Day 19 drop in temperature (early AM)
Day 20 thermal shift, temperature up (early AM)
Day 21 temp up still (Early AM)
Day 22 temp up still (now with three days above coverline)

Didn't manage to DTD on day 19.

There is a chance I might have conceived DTD on day 18 isn't there?

Hoping.....