Hi Sam. Thanks for your suggestions.
I just measured the 2 batteries. Both showing 24.9V on one voltmeter and both 25.0V on another. When I plug them in to the battery box and check my handlebar mounted power analyser it shows 50.19V. So no voltage drop that I can see. Or is this only a valid test under load? Please excuse my ignorance.
Resistance I think I can measure with my multimeter, I will give that a try and report back.
I have a couple of little temperature probes which I can wire up. I could tape 2 of them to the crimp connections and go for a ride.
It's so cold at the moment in the UK, not fun cycling weather.