my old Davenset charger had a jumper that you could swap between 6 and 12 volts, the workshop Sealey only does 12v but has a boost to use on cars while still in situ to aid starting. I looked for a new one recently, there are a few chargers out there but not many 6v. the intellegent ones are normally like the Optimate type that will keep the battery charged whilst still on the bike but generally they only keep the charge to the same as when its plugged in. so pays to go on a run to get full charge to maintain that.

I have an Accumate 6/12v but also a trusty vintage 1950s lump that does anything that’s asked of it.
Try Ebay, type in “Vintage battery charger”. If it’s working, it’ll probably work for ever!

there are 6v chargers on google or

https://www.youtube.com/watch?v=UVkv_oVtG3w

Thanks for the replies so far.

My C15 is positive earth, so does it matter if the bracket around the coil is bolted to the frame.

New 3 coil spring now ordered!