You state the "contractors" are refusing.
Whoever took out the warrantyis responsible for dealing with this issue. It may be Catalyst.
As it would seem they will probably both refuse to do anything, your only option now would be to contact your warranty provider.
From what you say in your post, I very much doubt a warranty claim would be refused and, at the very least the brickwork should be raked out to a depth of 25mm and repointed using 1:3 or 1:4 sand : cement mortar.
I also envisage the weak mortar problem not being to just your home but throughout the development and the cost of remedial works throughout the development could well bankrupt either the contractor responsible or the developer liable.
In addition, this being the case the warranty provider may also attempt to deflect and wriggle to avoid any responsibility as the claim could be very expensive.
You might find my
articles on weak mortar on my blog website worth a read.
From the experience of others, you may have a long fight on your hands.