Thank you Tony for your response.
Most likely I would ask someone to replace part/boiler (recommendation welcome - I am in Catford SE6) but I would like to diagnose problem first as don't want spend a lot of money for a new boiler if eg. fixing could be done for tenth part of it. As the leaking...
I don't know how to upload the image so i will post the link to the post where needed photo is attached
//www.diynot.com/forums/viewtopic.php?t=235119
My problem is that black metal part is leaking - I assume it is rusted as it is mini hole at the front of it and the warm water is...