resting at 12.4V

That isn't great Simon - a good wet 12V battery should be 12.7 - 12.8V resting after a charge (AGMs & Gels are higher than wet). 12.4 volts is around 70 - 75% charged & I would have expected more after charging with a modern 3 stage charger (note that old transformer + rectifier chargers may not fully charge a battery).

A decent make gel or AGM battery may have a higher CCA rating than a wet battery & big twins need a lot of cranking current compared to a four. I used Varta wet batteries for Mrs B's R65LS for decades (she has owned it for 40 years next January), but recent purchases have not lasted well - they used to do 5-6 years, but recently only 2 years. Looking for alternatives i found that the same size AGM batteries had nearly double the CCA of current Vartas - the no-name item I bought last spins the engine noticably quicker than with the Varta wet battery & is currently 2 YO and still going strong.

AGMs don't like deep discharges, though, and the battery paperwork warned against letting the resting terminal voltage drop below 12.5V or the battery may not recover - a monthly recharge reqime over the Winter with an Accumate seems to have kept this one good. All my REs (Himalayan, Interceptor, Mrs B's Scram) and Mrs B's V7 Special have AGMs and all are holding up well - the Interceptor & V7 are 6 YO now.

Seeing your pics above reminded me of a "foible" with my Mille. The HT leads were routed through a moulded-in grommet or eylet on the underside of the carb mounting rubber boot. These rubber boots move noticably when the bike is at tickover with the throttle closed - the suction against the closed carb causes the boots the flex. The coils are mounted side-on under the LH side panel & on my bike the angle of the LH cylinder HT lead through the grommet acted like a one-way mechanical clutch (like the tension bar on sealant guns) - the pulsing movement of the grommet pulled the LH cylinder HT lead tight to the point that it pulled the lead out of the push-fit coil. Cue increasingly bad misfires and backfires. Easily cured when you have sussed out the problem, though - just push the HT lead back into the coil at the side of the road & then later shave the sharp inner corners off the grommet after taking the lead out & lubricate with a smear of silicone grease on re-assembly. Didn't affect the RH cylinder, as there was enough slack in the longer lead to that side not to pull the lead tight - both coils are under the LH side panel.

Nigel B.