If I stick with the 3m lengths, is an straight overlapped joint ok, or
better to aim for a wrap around like so?

I just overlapped mine, but put a small bulge in (about 5mm or so), so the
water would have to go uphill to run underneath. I also used a substantial
overlap. No sign of any water ingress.

Working with lead is great. It must be the nicest material I've ever used.
It cuts easily. It bends easily. It stays exactly where you put it.

One tip. Make sure you apply patination oil before installation. I generally
did, but forgot one piece, which does look significantly more manky than the
other bits.

You have the choice of sealing with mortar or lead silicone. I went with the
lead silicone as it is less work, as you don't have to mix up the mortar and
carry a bucket up there.

Christian.