From my understanding you do not have to give them an opportunity to rectify the work, you have already given them one. Its known as 'loss of face', where you have no confidence in their ability to complete the job satisfactorily.
How much do you owe them and how much have you paid so far?
I'm thinking that if you've only paid them 20% you are at least the right side of this financially. You're perfectly within your rights to account for loss of time and damage to the house as a result of their incompetence.
Add it all up and send them the invoice for that and agree to consider it settled if they agree to surrender legal title to all the materials they've fitted (boiler, pipes etc) then leave the job without charging you anything else.
The work is worthless IMO. The boiler has a value as do some of the other components they fitted.
Then when they've finally realised that they would be better off doing that then get a decent plumber to refit everything. Much of the pipe work will be scrap. The boiler will be OK as will a few other bits. You'll be about right on the cost, maybe financially advantaged by a few quid.