We have a timber suspended floor in our victorian rental house which is caving in.

Hi. You can concrete the area like you say. Or you could lift the old floor up check joists (sound rot anyway) remove them. Check the plate that the joists sit on replace if needed and replace joists and new flooring over the top. Concrete is obviously the strongest choice. But just remember about pipe work and cable under the floor in that area might need moving.

When renovating or making repairs to older properties i would always aim to repair/replace with like for like where possible. They were designed and built that way for a reason and have more than proven the test of time. Concrete could be the answer although there are many cases of concrete floors being laid in this type of property causing damp issues. Please take the time to do your research and read up fully on this subject to help you make an informed decision.Pros and cons, right way, wrong way etc. Ultimately this problem and the cost of any work should surely be down to your landlord to inquire about. Kind Regards