Repair or Replace? – £16,400 Section 20 bill proposed for new door

The rear door of one of the Brandon tower blocks (Walters House) has fallen off, as the metal boxing that secured it has completely rusted away. In total there are 18 of these identical door units on the estate, but only this one door has suffered such a failure. The residents believe that a major contributing factor has been an ongoing water leak within the block, which our TRA have been continually reporting to the Communal Repairs team for over six years. The source of this leak has never been identified, or the issue resolved.
The LBS contractor, Silk & Mackham have proposed a replacement of this door & frame costing £16,481.14, and Southwark have issued Section 20 notices to affected leaseholders amounting to £247.22 each.
However it appears that no consideration is being given to effect a simple repair to replace the corroded steel boxing (60mm x 100mm x 1800mm)  that is anchored to the cast concrete wall of the building, as pictured below. A google search suggests that the cost of the required 6mm steel box section would cost in the region of £80 – £120. Obviously there would be labour costs to be added as well. The existing door, which is undamaged (save for needing a new pull handle)  and still on-site, could then be reattached.
At a time when money is short it seems incredulous that Silk & Mackham are proposing to replace the whole unit, rather than carry out a simple repair?
Our TRA have raised this issue with our ward councillors, Cllr Natasha Ennin, Cllr John Batteson & Cllr Youcef Hassaine in the hope that they can support the Brandon leaseholders on this issue.

Images before door fell off the frame

Images of the box steel section that could be replaced