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Thanks for the further thoughts and I understand the logic - I bought a new gel battery about 4-5 years ago - it reads around 12.6 volts when open circuit and my mains charger tells me that it reaches full charge. It also holds a charge very well - having ridden some distance without the generator working without any issues.
I also took your advice and fitted a relay to the charging circuit so the regulator reference voltage is always at the battery voltage and not subject to the various voltage drops caused by poor connections at junctions and switches. The battery voltage is stable at 14.85 volts above about 2500 rev/min.
However, I have now investigated the various voltage drops in my wiring loom and I lose about 1.5 volts when I switch the lights on (the difference between battery and regulator reference lead) My multi-meter tells me that the vast majority of this voltage drop occurs over the main key switch and the ignition key switch. I have pulled both of these off the bike and have been overhauling them. Both had variable resistance between the main battery feed wire (white lead) and the main 12 volt lead (brown wire). I have also fitted a Koso digital LED voltage meter to keep an eye on the charging circuit.
I will post some pictures of how I got the main (under seat) key switch apart and back together as it is not straightforward. The ignition switch is simpler but needs care to avoid breaking it when taking apart. Both switches cleaned and serviced and now showing zero ohms resistance - I will fit them back on the bike at the weekend and hopefully measure some reduced voltage drops.

This is the relay fitted to the charging circuit



And this is the Koso voltmeter - spark system and lights off

Nice !! Found a voltage drop and went after it. I like it !! 1.5 volts across a switch is a lot.

I fitted a similar voltmeter to my old Intruder as they were renowned for having charging issues (and yes - I had to replace the stator and RR on that one). I hooked the relay up via a switchable wire somewhere in the headlight bucket and ran the relay straight off the battery (which pretty much does the same thing as your version). Originally I ran the voltmeter straight off the switchable wire but got a massive drop in the voltage reading when I switched on the driving lights / hi beam. Straight off the battery is the best place.

I've also pulled apart most of the locks on mine and swapped the pins around so they all operate on the one key. My brother got a new ignition switch some years back, and the keying on that was still like new, so I matched up all the others to suit it. Going from 3 keys to 1 was nice. Yes - the power switch lock is a tricky one to disassemble, but the helmet locks were a piece of cake as you can unscrew the back covers and get straight in there. The tank lock is pretty easy too - although there is a fair bit of disassembly work to get down to the barrel.

Nice work all round.

Outstanding job of installing the relay for the R/R sense circuit. Thanks for the professional quality photos. I think you have solved the problem of the stator failures. That 1.5 volt drop to the sense circuit with the headlight on made the stator put out a lot of current attempting to charge an already fully charged battery. Having the headlight on increased the load on the stator, causing even more current flow through the stator output leads.

If you post photos of how to dismantle and clean the main switch and ignition switch, it will be a tremendous help to many 1300 owners. Dirty and excessively resistive contacts inside these two switches are a common problem. It possibly causes many stators to fail exactly as yours did.

Some owners have claimed success in eliminating voltage drops in the switches by spraying Deoxit contact cleaner into the switch and operating the switch to clean the contacts.

There are 2 other problems caused by too much resistance in the main and ignition switches. The voltage to the coils goes through both switches and the kill switch before reaching the coils. There is sometimes a 4 volt drop. That means 8 volts available for the ignition coil primary circuit. The situation is worse if the voltage dropping resistors for the coils are used. The resistors drop the voltage by 50%. Thus, 8 volts in provides 4 volts out to the ignition coils. That is why many 1300's become very difficult to start.

The other problem is that the lights will not receive 12 volts, creating a hazardous situation for the rider. Dim brake lights can be hazardous to one's health.

Pity someone can't come up with a small unit that indicates charge that would fit into that "spare" idiot light place on the far right of the cluster if you get my meaning ???, perhaps 3 small LEDS .. a red an orange and a green that light up depending on the voltage supplied.