I'd like to have a bash at lead filling on my '66 Daimler resto but have little (no) idea what I need, would my plumbing torch/solder/flux suffice or do I need different gear?
That's a lot like sweating a lead to copper pipe.
An old plumbing skill, if you need/want a sweat pad (mole skin pad) and tallow you'll only get them from good plumbers merchants.
I'm a little confused by your write-up (which is brilliant trodat) I've always considered 'tinning' as pre-soldering an area or wire, not fluxing. I'd call that fluxing?