Intermittent Alton starter

Battery? Or possible the main battery leads connectors have gone high resistance.
CTEK charger NOT to be trusted.

If Al's suggestion brings you no result try and bypass the solenoid assuming one is fitted on the Alton system.

Tighten the battery terminals?

Your CTEK smart charger is not able to take your battery to it's full capacity if you keep the battery on the bike while charging.

The Zener diode, or aftermarket reg/rec (if fitted) tend to prevent a smart charger from being able to condition a battery properly as the regulator setpoint won't let the battery charge fully.

The result is the battery life will be massively shortened.

When you replace your dead battery, try charging your new one off the bike and see if this helps matters.

The battery used on this bike WAS 18Ahr but it is no longer! You can try to charge it how you like but you will NOT get 18Ahr from it again. Batteries are biscuit barrels ie you refill them when you eat the biscuits! BUT you are refiling them with the biscuits in the packet, eventually the biscuit barrel is full of empty PACKET it cannot hold biscuits.
The only way to to be able to say the battery is FULLY charged is to charge it properly for a good time, then discharge it (as far as you can) to see what Ahr you had in it. (this is a tricky job)
Your 18Ahr battery MIGHT only have 10Ahr left.
Do NOT believe automatic lamps on modern battery chargers.
In this particular case just one terminal having gone high resistance will stop the starter/battery.performing.
And you no longer have 18Ahr!

Yuasa is a very good battery but of course expensive, but can you be sure you are dealing with the original Yuasa?
Motobatt seem to be good and usable.
Exide was a good maker of these things but the name has been stuck on another 'biscuit' barrel.
Stan's switch is ok but you do not need a centre off, a changeover switch will do. Adding switches and connectors to the bike will increase the chance of failure so pulling out the fuse and connect the battery charger just to the battery wire will suffice.

I have been taking an interest in this topic from the beginning and was mildly alarmed to read about battery minder/chargers etc not doing what I expect it to be doing such as prolonging my bike batteries life especially the large one that turns my Commando over!
Intrigued by Grant Tiller's post regarding this subject below.
 I took a reading of my battery voltage as it had been on charge via the Optimate 4 charger since coming back from the Manx last Saturday, reading fully charged was 13.45v.
Removed earth lead from battery, replaced the Optimate to the battery to then see if any more volts could be witnessed on the Avo 8 meter, in fact the meter reading was fractionally less by .15v!
As a back up I took all readings with a quality digital meter which gave a similar outcome give or take a decimal point or two.
This set up is over 18 months old, re'conned dynamo new electronic regulator, new battery new Optimate, gives me a bright headlight and a healthy reading on the ammeter, spark at the plugs,all looks good.
Yesterday I sent Optimate an abridged version of my concerns about what I am reading on this forum topic. As yet no response so far.

""Submitted by Grant Tiller on Tue, 10/09/2024

In reply to Battery charging by Rob Bradley

Because the reg/rec is…

Because the reg/rec is controlling the maximum voltage whether your charger is connected to the battery terminals or not.

You smart charger doesn't know there is a reg/rec there, so will trickle charge for forever which dries out and sulfates the battery rather than conditions it like it tells you in the instruction manual.

Imagine a kitchen sink with two overflows one above the other - the top one (the charger) will never get wet because the bottom one (the reg/rec) is controlling the height of the water (the charge)""

Since it fails on the first push of the button, then will likely start on the second, I suppose the obvious thing to try is a jump from a car battery on a cold morning. That'll tell you whether it's the battery or something else.
But it does sound like a battery on it's way out - looks like it's fully charged but not delivering enough juice. Can go on for many months like that before it calls it quits. Time to refit the kickstart, or practise bump starts.

Your battery sounds fine. I have just changed the brushes in my Alton for the second time in 10 years. Petrol from tickled carbs can find it's way in and shorten brush life, a smear of silicone sealant can help on reassembly. Replacement brush sets are available on Ebay. Easy enough to check and change. Only other electrical issue I've had is cable coming loose on solinoid so no biscuits required only for the cuppa when the job is done :-)
Best regards, Al.

I mentioned the relay as a potential problem due to the internal contacts taking the high amperage during starting procedures therefore arcing produces pitting and this in turn can lead to a high resistance between the two contacts.
If you can dismantle the solenoid it should be possible to emery the four  faces and clean them up sufficiently for you to start the motor and prove a point.
It goes without saying that all of the connections need to tight to and from the heavy  starter cables.

ALTON NORTON COMMANDO ELECTRIC START CONVERSION MITSUBA STARTER - BRUSH PLATE from Nipponautomotive.
I have also replaced the solenoid (relay) with a generic Japanese one but also first check the thick cable connections haven't vibrated loose or corrosion hasn't worked it's way in there.
Your battery sounds fine.

Best regards, Al.