What an unfortunate and shocking incident.
I'm amazed the Galvanised tank itself fell down through the ceiling, it's usually situated on top of strong timbers, or was it just the water that escaped?
Were you actually inside the house when this happened?
I can see from the bathroom image above, that your ceiling boards are the original fibre-board panels that were fitted inside most of the BISF houses in Birmingham. Unfortunately, due fibre-boards composition, it's very porous, and extremely weak when saturated.
When originally built, it was specified that fibre board should only be used at ground floor level, as it was not sufficiently fire resistant to be used upstairs, but this specification was ignored in much of the Midlands.
Are you currently living inside the house?
If this were my property, I would consider having a combi boiler central heating system installed, as this would remove the need for a water tank in the loft altogether. If that's not an option, then the tank needs to be repaired or made safe again, which is outside my area of knowledge.
The next stage would be to allow the house to dry out if possible and to then have new drywall fitted to affected walls. I would also take this opportunity to insulate the loft too, as I can't see any insulation in the photos.
What I can't tell, is whether your walls are currently covered in drywall, or original hardboard. If they are covered in hardboard, it might be better to replace each affected wall in drywall.
The rest of the floor damage appears to be repairable.
A good general builder or drywall installer/ plasterer should be able to provide a quote.
I'd love to hear a bit more about what exactly happened, and what your plans or thoughts are so far.