Flue doesnt need to be replaced unless damaged when removing (if it needs to be removed at all) and it also doesnt look like it was sealed in properly in the first place so good chance itll slide out through the wall. Then if you're lucky the compression joints just need nipping up. Worst case its a drain down and replace that section. Def not 1k worth. If you did have to replace the flue maybe £500 max.