It stands to reason that as soon as ISKY 1 stopped creaking, ISKY 2 would start ticking. As with the apparent source of the noise from the purple bike, the ticking seemed to come from the bottom bracket of the orange bike.
Hoping to benefit from past experience, I started my hunt for the source of the tick with the rear hub. It was in a better state than the one on ISKY 1 had been. It still needed to be cleaned and re-greased. The freewheel runs quieter now. But the ticking persisted.
The next step was to remove the cranks and take a look at the bottom bracket. The bearings looked fine, apart from needing a good clean and some fresh grease. I did discover this though.
This disc spring sits on the drive-side crank spindle.
There is a new disc spring in place of this cracked one now.
ISKY 2 is still ticking though. But, unless I am imagining things, not as loudly as before.
The optimist in me thinks the ticking will just fade away.
The realist in me knows that there is more work to be done.
Old Roots, New Routes 
good detective work. strange sounds get louder during long rides!