On Friday, 12 August 2016 02:14:04 UTC+1, bob wrote:
replying to Chris Holford, bob wrote:
boiling water removes caustic soda which gone hard, along with a plunger

I'd think after 4 years he'd have solved it by now.
Try this:
http://wiki.diyfaq.org.uk/index.php/Home_owners_hub


NT