Small brazing job for someone in Basingstoke.

[zeppelin builder]

That's quite a crack, all the way around?

What caused it, I wonder?

if the brazed joint is overheated the brass gets in to the crystalline structure of the steel and seriously weakens it its
the cause of most motorcycle /cycle frame defects
you can see when you dissasemble old brazed assemblys the tube crumbles as you twist it out and it has a look like ragu beef if you polish the surface with the brass randomly distributed
id drill a 2mm hole at the end of the crack at the edge of the braze. tig round to the hole as to avoid brass in it then care fully fill the hole with a dollop of brass after cleaning

 

[striker]

I reckon that's been caused by over extending the seat post. The bottom of the seat post looks to have been extended beyond the top tube joint.