Once you know the cost, my suggestion would be to write to Miller CEO Keith
"Our mission and values are to deliver world-class levels of service to our customers" Miller.
You should tell him your problem, how long it has been going on and , how much it will cost to put right.
Whilst he can under the warranty wriggle out of any responsibility, Miller may have a liability under the Defective Premises Act.
I would inform him that unless he reinburses the leaseholders with the full cost of the repairs, you will be
taking action in the County Court to recover the cost of the work.
It is easy to do and there are no legal fees involved for you. You may not win the case but for around £110 plus the hearing fee of around 10% of the amount claimed, (You get the fees back when you win) you can have a day in court and invite all the press you like.
My guess is Miller will pay up before the court hearing!