I suspect the seat tube was already rusted or maybe there was a hole caused by rust in the tube. When I tried to get the seat post in a bit lower I put my weight (only 70 kg's mind you) on the saddle (it was moving free but wouldn't go lower) and heard a strange noise. It must have been the hole or crack getting bigger. At first you couldn't tell as the paint was still intact but I scratched the paint to see if it really had cracked.
When I collected the bike I noticed it had a repaint had the decals are not right. So, like I wrote before it was a calculated risk and at the price I paid I cannot complain. That's also why I want to get the complete frame stripped off the paint to see what it looks like. After that the most important thing is to find a welder with enough experience to get it properly repaired. I have adjusted my plans: first ride will be 2018 now.